Introduction
In 2011, the largest monolithic carbonate gas reservoir was discovered in Gaoshiti-Moxi in the central Sichuan Basin, China, where the major pay is the Sinian Dengying Formation. This discovery, after the Sinian gas reservoirs in Weiyuan and Ziyang discovered in the southwest Sichuan Basin in the 1960s, demonstrates a great potential of petroleum exploration in deep and ultra-deep Proterozoic carbonate formations of the Sichuan Basin[1,2,3]. But the exploration and study on the Dengying Formation so far have been restricted in the central and southwest Sichuan Basin, while other areas in the basin have low degree of exploration and unclear situation of reservoir development. Due to more exploration activities aiming at the Sinian Dengying Formation in the north Sichuan Basin, we found many bituminous veins in the north part of the Longmenshan Mountains and the front of the Micangshan Mountain, and high-graded porous reservoir rocks during field geologic surveys in north Sichuan and its surrounding areas. These may indicate possible Dengying reservoirs in this region.
As per preceding studies on the Dengying Formation in the central and southwest Sichuan Basin, there are three viewpoints on the origin of Dengying Formation reservoirs, i.e. supergene karstification in the Tongwan stage presented by Mo et al. and Luo et al.[4,5], penecontemporaneous exposure and corrosion presented by Chen et al.[6], and dolomitization of micro-biolite presented by Song et al.[7]. In field geologic surveys, we found some differences between Dengying reservoirs in the north Sichuan Basin and its surrounding areas and those in the central and southwest Sichuan Basin. We inferred that Dengying reservoirs in different parts of the Sichuan Basin may be different in origin. In this study, we try to find out the characteristics, origin, main control factors and distribution pattern of reservoirs in the second and fourth members of the Dengying Formation in the north Sichuan Basin and its surrounding areas, in the hope to provide new information for petroleum exploration in the Dengying Formation, north Sichuan Basin.
1. The geologic setting
The region of interest (ROI) lies between north Sichuan Province and south Hanzhong City in Shaanxi Province. Tectonically it comprises Micangshan uplifted zone, northern Sichuan low-flat belt, and northern part of Dabashan faulted folded belt (Fig. 1). In this area, the depression in the west transited to folded zone in the east, indicating enhanced tectonic movement.
Fig. 1.
Fig. 1.
Location map of the region of interest.
The Dengying Formation deposited after the Doushantuo Formation in the Sichuan Basin is a package of transgressive retrogradation sedimentary sequence composed mainly of shoal carbonate platform deposits[8] (Fig. 2). The Dengying Formation in the ROI is divided into 4 members in accordance with outcrops observation and drilling data[9]. At the depositional stage of the first member (Deng-1 Member), massive transgression occurred and thus an epicontinental sea appeared in upper Yangtze region. In the Sichuan Basin at this time, the sedimentary basement was relatively flat and there was little change in water depth, and algae-lean micritic to crystalline powder-scale dolostone deposited in the ROI. At the depositional stage of the second member (Deng-2 Member), the ROI was in a platform-basin environment due to sea level drop and tectonic movement, algae-rich mound-shoal complexes occurred from platform margin to internal platform, which are comprised of stromatolitic dolostone, thrombolitic dolostone, micritic to crystalline powder-scale dolostone, and some dolarenite and dolomitic karst breccia. At the end of Deng-2 deposition, the Deng-2 Member was uplifted and denuded due to Episode-I of the Tongwan tectonic movement, forming an unconformity between the Deng-2 and Deng-3 Members. At the depositional stage of the third member (Deng-3 Member), thin dark mudstone and siltstone deposited in most of the Sichuan Basin due to rapid transgression. Owing to the impact of Hannan Ancient Land, in Hanzhong-Ningqiang in northern ROI settled purple and khaki moderately to thinly laminated mudstone, siltstone, and pelitic siltstone intercalated with conglomerate of shoal tidal-flat facies. At the depositional stage of the fourth member (Deng-4 Member), sea level dropped and water depth lessened. The ROI had similar sedimentary framework to that during Deng-2 deposition. The lower part of Deng-4 Member is composed of micritic to crystalline powder-scale dolostone with small content of algae. The middle and upper parts are composed of stromatolitic dolostone, thrombolitic dolostone, dolarenite, and micritic to crystalline powder-scale dolostone, with striped siliceous dolostone occasionally. At the end of Deng-4 deposition, the Deng-4 Member was denuded due to Episode-II of the Tongwan tectonic movement; an unconformity occurred between the Cambrian System and the Dengying Formation in the platform. In addition, there was a pull-apart ocean trough, deep in the north, shallow in the south, steep in the east, and gentle in the west, in Mianyang-Changning of west Sichuan at the later stage of Dengying deposition[10,11,12]. The platform margin, extending in the north-south direction along the transition zone between the ocean trough and platform, had abundant mound-shoal complexes deposited, and experienced intense corrosion in the later stage, so this area is the favorable site for reservoir development and hydrocarbon accumulation.
Fig. 2.
Fig. 2.
Composite stratigraphic column and sedimentary facies of the Upper Sinian Dengying Formation in the ROI.
2. Reservoir features and differences
Controlled by different factors, Deng-2 and Deng-4 reservoirs in the ROI have different features (Table 1).
Table 1 Features and main control factors of Deng-2 and Deng-4 reservoirs in the ROI.
| Layer | Rock type | Pore space and interstitial materials | Petrophysical property and connectivity | Reservoir distribution | Control factors |
|---|---|---|---|---|---|
| Deng-2 | Thrombolitic dolostone, stromatolitic dolostone, and dolomitic karst breccia with grape lace-shaped cement | Isolated dissolved pores and dissolved cavities extended parallel to the layer among algal frame, and cavities in grape lace-shaped cement in dolomitic karst breccia, often partially filled with multi- phase dolosparite, with no bitumen observed. | Poor connectivity, low porosity, and low permeability | Middle and upper Deng-2 | Mound-shoal complexes + supergene karstification |
| Deng-4 | Thrombolitic dolostone, stromatolitic dolostone, crystalline powder-scale dolostone, and dolarenite | Dissolved pores, dissolved cavities, intercrystalline pores, intercrystalline dissolved pores, and intergranular dissolved pores among algal frame, partially filled with bitumen and some quartz. There is hardly dolomite filled in pore space. | Good connectivity, low to moderate porosity, and low permeability. High porosity and moderate permeability locally. | Middle, upper and top Deng-4 | Mound-shoal complexes + burial dissolution |
2.1. Reservoir-rock types
There are two main types of rocks, i.e. dolostone and fine- grained clastic rocks, and minor dolomitic karst breccia and siliceous dolostone in the Dengying Formation of the ROI. Algae-rich dolostone takes absolute majority in the reservoir rock. Reservoir rocks in the Deng-2 Member are mainly thrombolitic dolostone and stromatolitic dolostone (Fig. 3a-3c) as well as some dolomitic karst breccia with grape lace- shaped cement (Fig. 3d). Reservoir rocks in the Deng-4 Member are mainly thrombolitic dolostone and stromatolitic dolostone (Fig. 3e-3h) as well as some crystalline powder-scale dolostone (Fig. 3i-3k) and dolarenite (Fig. 3l).
Fig. 3.
Fig. 3.
Major types of reservoir rocks and pore space in the ROI. (a) Gucheng section, Wangcang, Deng-2 Member, thrombolitic dolostone, inter-algal frame pores completely filled with multi-phase dolosparite, polarized light; (b) Gucheng section, Wangcang, Deng-2 Member, stromatolitic dolostone, intra-laminal dissolved pores partially filled with multi-phase dolomite, polarized light; (c) Shuimo section, Wangcang, Deng-2 Member, thrombolitic dolostone, inter-algal frame dissolved cavities partially filled with dolomite, poor connectivity; (d) Yangba section, Nanjiang, Deng-2 Member, dolomitic karst breccia, residual cavities in grape lace-shaped cement; (e) Hujiaba section, Ningqiang, Deng-4 Member, thrombolitic dolostone, inter-algal frame pores partially filled with bitumen; (f) Hujiaba section, Ningqiang, Deng-4 Member, thrombolitic dolostone, inter-algal frame dissolved pores partially filled with bitumen, polarized light; (g) Hujiaba section, Ningqiang, Deng-4 Member, stromatolitic dolostone, intra-laminal dissolved pores and cavities partially filled with bitumen; (h) Hujiaba section, Ningqiang, Deng-4 Member, stromatolitic dolostone, intra-laminal dissolved pores (generated due to dissolution of clastics-rich laminae) partially filled with bitumen, polarized light; (i) Gucheng section, Wangcang, Deng-4 Member, crystalline powder-scale dolostone, intercrystalline dissolved pores of non-selective dissolution origin partially filled with bitumen and quartz, polarized light; (j) Gucheng section, Wangcang, Deng-4 Member, crystalline powder-scale dolostone, intercrystalline dissolved pores and dissolved cavities of non-selective dissolution origin partially filled with bitumen and quartz, polarized light; (k) Gucheng section, Wangcang, Deng-4 Member, crystalline powder-scale dolostone, dolomite intercrystalline dissolved pores, weak quartz dissolution; (l) Bashan section, Zhenba, Deng-4 Member, dolarenite, non-selective intergranular dissolved pores occasionally filled with bitumen, polarized light.
2.2. Pore space
As per macroscopic and microscopic observations, pore space in the Dengying Formation of ROI is composed of secondary pores and cavities. Fractures connecting pores and cavities, can improve reservoir permeability.
Pore space in the Deng-2 Member is composed of intra-stratal isolated inter-algal frame dissolved cavities and cavities in grape lace-shaped cement in dolomitic karst breccia. Thrombolitic dolostone and stromatolitic dolostone are very rich in primary inter-algal frame pores, which were originally larger than 1 mm in diameter, but became much smaller due to cementation, and later were enlarged by dissolution to form inter-algal frame dissolved cavities of 5-15 mm, even above 20 mm in local parts, in diameter. These dissolved cavities are usually partially filled with multi-phase dolosparite and some quartz, but has no bitumen and poor connectivity (Fig. 3b, 3c). Cavities in grape lace-shaped cement are residual pore space in corroded gullies, dissolved fractures, and dissolved cavities in dolomitic karst breccia partially filled with grape lace-shaped cement, granular dolosparite, and quartz, with diameter of generally 0.5-2.0 cm and larger than 10 cm at maximum (Fig. 3d). Grape lace-shaped cement is fibrous dolomite with 2D lace shape and 3D grape shape. The cement may originally be generated by cyanobacteria[13] or the product of supergene karstification[14,15,16]. Cavities in grape lace-shaped cement are a unique kind of pore space in the Deng-2 Member. There are no such cavities in the Deng-4 Member.
Pore space in the Deng-4 Member is composed of intra-laminal inter-algal frame dissolved pores and dissolved cavities as well as some intercrystalline pores, intercrystalline dissolved pores, and intergranular dissolved pores. Inter-algal frame dissolved pores and dissolved cavities in the upper member extend parallel to laminae and are partially filled with bitumen or some quartz largely, but with hardly any dolosparite (Fig. 3e-3h). This is different from the Deng-2 Member. Intercrystalline pores and intercrystalline dissolved pores mainly occur in crystalline-grained dolostone and crystalline powder-scale dolostone with residual grain structure; most of these pores are partially filled with bitumen and quartz (Fig. 3i-3k). Some intergranular dissolved pores of 0.05-1.00 mm in diameter can be observed in dolarenite; these pores are generally filled with bitumen (Fig. 3l).
2.3. Reservoir properties
As per conventional physical properties tests of samples, Deng-2 samples have a porosity of 0.33-5.12%, on average 2.15%, and all below 6% (Fig. 4a); a permeability of (0.000 1- 6.030 0)×10-3 μm2, on average 0.318×10-3 μm2 (Fig. 4b). Deng-4 samples have a porosity of 0.35-21.69%, on average 4.95%. Samples with the property of 2-8% account for 71% of total; 11.3% of samples have porosity above 10% (Fig. 4c). Deng-4 samples have a permeability of (0.000 1-20.900 0)× 10-3 μm2, on average 1.177×10-3 μm2 (Fig. 4d). Dengying reservoirs in the ROI are of middle to low porosity and low permeability, but Deng-4 reservoirs, with more high porosity samples, have better properties than Deng-2 reservoirs. Some Deng-4 reservoirs are of middle to high porosity and middle permeability. Deng-2 and Deng-4 reservoirs are both of low permeability on the whole. Pores in Deng-2 reservoirs are often partially filled with multi-phase dolosparite and poor in connectivity, so the reservoir is low in permeability. Pores in Deng-4 reservoirs are filled with bitumen and quartz, so some pore-throats are blocked, causing low permeability.
Fig. 4.
Fig. 4.
Histograms of physical properties of Dengying Formation in the ROI (N indicates the number of samples).
3. Genetic differences of the reservoirs
3.1. Sedimentary facies control
The major part of the Dengying Formation in the ROI is carbonate platform facies (Fig. 2b, 2c), which includes two subfacies, i.e. platform margin and restricted platform. The former is composed of platform marginal mound-shoal and inter-mound-shoal bottom land; the latter is composed of dolomitic lagoon and intra-platform mound-shoal. As per the study, good reservoirs tend to occur in platform marginal mound-shoal, followed by intra-platform mound-shoal in restricted platform. Inter-shoal bottom land in platform margin is also promising. Dolomitic lagoon in restricted platform is unfavorable for the development of reservoir (Fig. 5).
Fig. 5.
Fig. 5.
Average porosity of sedimentary facies in the Sinian Dengying Formation in the ROI.
Two facies, i.e. platform marginal mound-shoal and intra-platform mound-shoal, have similar sedimentary features. These two facies both occurred at structural highs (topographic high in platform margin or inside platform), where strong hydrodynamic conditions and abundant nutrition were conducive to particle accumulation and cyanophyte (bacteria) growth. As a result, thrombolitic dolostone and stromatolitic dolostone deposited to form topographically uplifted mound-shoal complexes. These mound-shoal complexes were rich in primary inter-algal frame pores, which contracted or disappeared locally (Fig. 3a) due to cementation of multi-phase dolosparite, but such facies still has relatively high porosity and permeability than rocks in other facies. Dissolution fluids in later diagenetic process tended to flow through these facies with relatively high porosity and permeability to form secondary dissolved pores and cavities. The differences between these two facies are that the former occurred in platform margin with stronger water power; thus, mound-shoal complexes, for example in the Deng-4 Member in Hujiaba section, are large both in single-layer thickness and cumulative thickness. The latter occurred locally in structural highs in restricted platform, where water power was weak due to the distance to open sea, consequently, mound-shoal complexes, for example in the Deng-4 Member in Shuimo section, Wangcang, are smaller in single-layer thickness and cumulative thickness.
Dolomitic lagoons in restricted platform and inter-mound- shoal lows extended widely in plain lows between mounds and shoals, where water power was weak because these bottom lands were sheltered by surrounding topographically uplifted mounds and shoals and thus micritic to crystalline powder-scale dolostone deposited. These fine-grained sediments were subjected to strong compaction and corrosion in the later burial process. Primary intercrystalline pores in them contracted largely and the residual intercrystalline micropores are mostly inactive pores. On the other hand, these pure sediments in inter-mound-shoal lows, which hardly contained algae and shale, might experience stronger recrystallization in the burial process to form crystalline powder-scale to finely crystalline dolostone. Crystal dolomite grains may become coarser and automorphic, improving effective porosity and permeability. As a result, intercrystalline dissolved pores may form due to the impact of dissolution fluids. In addition to fine-grained carbonate rocks, some shale and scattered algae also exist in the dolomitic lagoons in restricted platform, which not conducive to later recrystallization and corrosion, are poorer in reservoir properties.
3.2. Control of supergene karstification on Deng-2 reservoirs
Due to the impacts of Episodes I and II of the Tongwan tectonic movement, the Dengying Formation in the ROI experienced 2 phases of supergene karstification, which gave birth to two weathering surfaces at the top of the Deng-2 and Deng-4 Members, respectively. Corroded gullies and dissolved fractures filled with clastics, and dolomitic karst breccia can be seen in Deng-2 and middle and upper Deng-4 on field sections and in drilling cores (Fig. 6), but no large dissolved caverns and grape lace-shaped cement, the typical product of supergene karstification, were found in the Deng-4 Member. This means supergene karstification had little impact on the Deng-4 Member. In contrast, intra-stratal and inter-stratal grape lace-shaped structures and residual cavities several centimeters to tens centimeter in diameter in grape lace-shaped cements are common in middle and upper Deng-2. Fibrous dolomite in grape lace-shaped cement has lower contents of Sr2+, Na+, K+, and Fe2+ than granular dolosparite forming in later two phases (Table 2); besides, grape lace-shaped cements show weak cathode-ray luminescence, indicating grape lace-shaped cement was formed in an opened, oxidized meteoric freshwater environments, whereas granular dolosparite was produced in an enclosed burial environment with high salinity. Based on field sections and geochemical features, we conclude that the occurrence of grape lace- shaped cement, similar to stalactites in modern carbonate dissolved caverns, may be related to Deng-2 supergene karstification caused by Episode-I of the Tongwan tectonic movement.
Fig. 6.
Fig. 6.
Dengying karst features in the region of interest. (a) Well MS1, Deng-4, 8 200.75 m, micritic dolostone, corroded gullies filled with breccia; (b) Well TX1, Deng-2, 2 535.02 m, micritic dolostone, corroded gullies filled with bluish grey mudstone; (c) Taoyuan section, Deng-2, dolomitic karst breccia with grape lace-shaped cement and inter-breccia cavities; (d) Lijiagou section, Deng-2, grape lace-shaped cement and crystalline dolomite cement, polarized light; (e) Lijiagou section, Deng-2, grape lace-shaped cement with weak luminescence, dark brown, crystalline dolomite with strong luminescence, shiny red cathodoluminescence.
Table 2 Electron microprobe analysis results of interstitial dolomite in Deng-2 dissolved cavities.
| Type | Well | MgO/ | SrO/ | Na2O/ | K2O/ | BaO/ | MnO/ | FeO/ |
|---|---|---|---|---|---|---|---|---|
| % | % | % | % | % | % | % | ||
| Grape lace-shaped dolomite in phase-I | MS1 | 21.30 5 | 0 | 0.00 9 | 0.00 5 | 0 | 0.00 2 | 0.01 2 |
| TX1 | 21.36 5 | 0 | 0.01 1 | 0 | 0 | 0.04 7 | 0.01 8 | |
| Foliated dolomite in phase-II | MS1 | 21.50 9 | 0.09 6 | 0.03 4 | 0.01 6 | 0.00 4 | 0.01 8 | 0.01 6 |
| TX1 | 21.91 4 | 0.06 4 | 0.06 2 | 0.02 2 | 0 | 0.06 0 | 0.03 2 | |
| Crystalline dolomite in phase-III | MS1 | 21.60 4 | 0.05 3 | 0.01 4 | 0.01 1 | 0.04 6 | 0.02 7 | 0.07 4 |
| TX1 | 21.44 6 | 0.06 8 | 0.01 9 | 0.01 7 | 0.00 9 | 0.01 2 | 0 |
Fig. 7 schematically shows the relationship between supergene karstification in the Deng-2 Member and Episode-I of the Tongwan tectonic movement. There are three intervals below the weathering surface, i.e. vertical vadose zone, horizontal undercurrent zone, and deep subcritical-flow zone, corresponding to upper Deng-2, middle Deng-2, and lower Deng-2, respectively. In the vertical vadose zone, there are nearly vertical corroded gullies and dissolved fractures generated by meteoric freshwater seeping down, but most of them are filled with clasts later. In the horizontal undercurrent zone, there are dissolved pores, cavities and fractures extending in a nearly horizontal direction because unsaturated meteoric freshwater flew in a horizontal direction due to the lift by phreatic water table. If such a zone occurs in platform marginal or intra-platform mounds and shoals, there may be more and larger pores. Meanwhile, solution breccia and intra-stratal and inter-stratal corroded gullies may also occur and then be partially filled with multi-layered grape lace-shaped dolomite. Although these dissolved pores may be partially filled with multi-phase granular dolosparite, there is still considerable residual pore space. In the deep subcritical-flow zone, granular dolosparite settled in saturated fluids, filling primary pores, resulting in poor reservoir properties.
Fig. 7.
Fig. 7.
Lateral comparison of Deng-2 sedimentary facies and supergene karstification model in the region of interest.
As per field surveys and sample tests, Deng-2 reservoirs in three outcrop sections were classified into three grades, i.e. grade 1 with porosity above 10%, grade 2 with porosity of 5-10%, and grade 3 with porosity of 2-5%. Reservoir rocks mainly occur in middle Deng-2, or the horizontal undercurrent zone formed by supergene karstification (Fig. 8). Where mound-shoal complexes are better developed, the reservoirs are better developed. The vertical vadose zone and deep subcritical-flow zone merely have some thin porous layers less than 0.5 m thick. Thus, supergene karstification in Episode-I of the Tongwan tectonic movement is crucial to the development of Deng-2 reservoirs. Reservoir rocks tend to occur in favorable sedimentary facies coinciding with horizontal undercurrent zone formed by supergene karstification in Episode-I of the Tongwan tectonic movement. Supergene karstification in Episode-II of the Tongwan tectonic movement may have some effects on Deng-4 reservoirs, but it is not the dominant factor.
Fig. 8.
Fig. 8.
Comparison of Deng-2 reservoirs in the region of interest.
3.3. Control of burial dissolution on Deng-4 reservoirs
Burial dissolution may improve carbonate reservoir properties[17,18,19,20,21,22]. Mazzullo et al.[23] suggested that CO2, H2S, and organic acid released by organic matter thermal maturation in the burial diagenetic process could give rise to secondary pores in carbonate reservoirs. She et al.[24] simulated dolostone dissolution by organic acids and concluded that organic acids could have strong dissolution to dolomite under deep burial. As per field surveys and microscopic thin-section observations, we think that although the Deng-4 Member in the ROI experienced penecontemporaneous exposure-dissolution, supergene exposure-dissolution and burial-dissolution, the pore space in reservoir rocks nowadays is largely the result of burial dissolution. With respect to interstitial materials, Deng-4 dissolved pores and cavities are filled with bitumen and some quartz (Fig. 3f, 3h-3j); this implies that hydrocarbon accumulation occurred shortly after the birth of dissolved pores and cavities in the Deng-4 Member. If dissolved pores and cavities were formed in the penecontemporaneous or supergene period, they must be filled with multi-phase dolomite cements, just like Deng-2 reservoirs (Fig. 3d). With respect to vertical distribution, Deng-4 reservoirs are concentrated from the middle to the top of the Deng-4 Member, with properties turning worse from the top down. This is different from the scenario in the Deng-2 Member, where corroded gullies and dissolved fractures in the vertical vadose zone at the top of supergene karst are usually filled with clasts (Fig. 6a); thus, reservoir properties at the top are worse than those in the horizontal undercurrent zone in the middle and upper part. Unlike the central Sichuan Basin, there are no large dissolved fractures and cavities in the Deng-4 Member in the ROI. In the central Sichuan Basin, large dissolved caverns and fractures were found in the Deng-2 and Deng-4 Members; this was verified by frequent circulation loss and drilling tool unloading in the process of well drilling. This means uplifting and denudation caused by Episode-II of the Tongwan tectonic movement had little impact on Deng-4 reservoirs in the ROI.
Preceding studies showed that the Qiongzhusi Formation overlying the Dengying Formation, is one of the best source beds in the ROI. It was mature enough to generate hydrocarbon at the end of the Cambrian Period and began to generate hydrocarbon on a large scale from the Permian Period to the Triassic Period[1, 25]. In the conversion from organic matter to liquid hydrocarbon and liquid hydrocarbon to gaseous hydrocarbon, a large amount of dissolution fluids including organic acid, CO2, and H2S were generated, providing dissolution material base for the Deng-4 Member in the burial process. At the early stage of hydrocarbon generation and expulsion, corrosive fluids originating in the Qiongzhusi Formation, along with compaction flow, moved downward into the Deng-4 Member and led to selective or non-selective dissolution. The former mainly occurred in algae-rich dolostone because it was hard to dissolve algae-rich constituents (algae-rich laminae and algae-bound constituents) and thus algae-poor constituents were dissolved selectively (Fig. 3f, 3h) to form nearly intra-stratal inter-algal frame dissolved pores, cavities, and fractures. The latter happened mostly in algae-poor dolostone (Fig. 3i, 3j) to form intercrystalline dissolved pores, dissolved cavities, and intergranular dissolved pores. In addition, thick Maidiping and Qiongzhusi source rocks with much higher content of organic matter resided in the pull-apart trough (slope-basin) in the immediate vicinity of the west platform margin[26], and the mass corrosive fluids they generated could flow laterally into the Deng-4 Member in the east platform. The model of dissolution is shown in Fig. 9. Deng-4 top and platform margin were much closer to source rocks and thus might be more intensely corroded by corrosive fluids to form dissolved pores. The corrosive fluids decreased with the increase of distance to Deng-4 top or platform margin; hence, there may be less dissolved pores in the area below Deng-4 top or far away from platform margin. Post-dissolution hydrocarbon charge inhibited carbonate precipitation; this is why almost no carbonate minerals have ever been found in Deng-4 reservoirs nowadays in the ROI. In this period, merely some SiO2 precipitated out from pore fluids and then filled dissolved pores and cavities.
Fig. 9.
Fig. 9.
Burial-dissolution model of the Deng-4 Member in the region of interest.
Formation temperature rose to above 160 °C after the Triassic Period due to increase of buried depth[27]. Crude oil started cracking to generate mass gas and bitumen as well as H2S and CO2[28], which pushed the gas-water contact downward in an enclosed high-pressure environment. As a result, the original reservoir space was enlarged. Besides, early pores might be corroded and enlarged by corrosive dissolved gas; quartz filled in original pores might be dissolved locally (Fig. 3k). Some pores were filled with bitumen generated by cracking, but residual pores still provide considerable reservoir space.
We examined Hujiaba section, Ningqiang, Gucheng section, Wangcang, Shuimo section, Wangcang, and Guimin section, Nanjiang and found that Deng-4 reservoirs are concentrated from the middle to the top of the member. Reservoirs at the top have the best properties. In spite of the existence of interstitial bitumen and quartz, surface porosity of the reservoir exceeds 15% at maximum. Reservoir properties get worse from top down and there are barely reservoir rocks in the lower part. In the lateral direction, Hujiaba section, Ningqiang was at the platform margin on the east side of the pull-apart trough (slope-basin), where Deng-4 mound-shoal complexes have the best reservoir properties. Gucheng section close to platform margin also exhibits good reservoir properties. Shuimo and Guimin sections inside the platform have only a limited number of intra-platform mound-shoal complexes and poorer reservoir quality (Fig. 10). This shows corrosion in the burial process is crucial to development of Deng-4 reservoirs, which are liable to occur in favorable sedimentary facies undergoing karstification in the burial process.
Fig. 10.
Fig. 10.
Reservoir distribution in the Deng-4 Member in the region of interest.
For the Deng-2 Member in the ROI, the overlying Deng-3 Member is composed of fine-grained clastic rock of hybrid tidal flat facies and the underlying Doushantuo Formation comprises purple pebbled sandstone, these two packages of formations basically have little organic matter and can hardly generate organic acids causing burial-dissolution to Deng-3. On the other hand, corrosive fluids generated by thermal evolution of organic matter in the Qiongzhusi Formation above Dengying Formation, were difficult to seep down to the Deng-2 Member. Therefore, burial dissolution had little impact on Deng-2 reservoir, this also may explain why there is no bitumen in the Deng-2 Member. In contrast, bitumen is common in Deng-2 pore space in the central Sichuan Basin, which may be related to high abundance of organic matter in Doushantuo argillutite and Deng-3 mudstone there. The oil generated by the organic matter would produce bitumen in the process of cracking, that would be left in the pores and cavities.
3.4. Genetic differences
As discussed above, the properties of Sinian Dengying reservoirs in the ROI are controlled by sedimentary facies and dissolution jointly. High-graded reservoirs commonly appear in mound-shoal complexes. But there are genetic differences between Deng-2 and Deng-4 reservoirs. With respect to Deng-2 reservoirs, supergene karstification in Episode-I of the Tongwan tectonic movement may have much larger impact than later burial-dissolution. As for Deng-4 reservoirs, limited pore space produced by supergene karstification in Episode-II of the Tongwan tectonic movement mainly functioned as the pathway for later corrosive fluid migration, and the reservoir properties depend more on the burial-dissolution. In the burial process, the multi-phase corrosive fluids not only produced secondary pores and cavities but also inhibited carbonate precipitation. Moreover, the hydrocarbon accumulation prevented pore space in Deng-4 reservoirs from filling by later diagenetic minerals. This is why Deng-4 reservoir properties are much better than Deng-2 reservoir properties. In contrast, large dissolved caverns and fractures in Deng-2 and Deng-4 reservoirs of the central Sichuan Basin are the product of supergene karstification[5].
4. Reservoir distribution and exploration targets
The study of reservoir genesis showed that the properties of Dengying reservoirs in the ROI are controlled by sedimentary facies and dissolution jointly. Areas of mound-shoal complexes undergoing supergene karstification or burial-dissolution are favorable reservoirs sites. The targets of exploration are deeply buried reservoirs with good properties.
4.1. Deng-2 Member
Deng-2 reservoirs in middle Dengying feature small single-layer thickness (of 1-10 m),large number of layers, and large cumulative thickness (of 20-60 m) (Fig. 8). Most promising reservoirs are porous and cavity-type dolomite reservoirs extending widely in an arc-shape region around Ningqiang- Guangyuan-Jian'ge, where mound-shoal complexes suffered dissolution and precipitating reformation of supergene karst fluids. The reservoir thickness ranges 40-60 m. For example, Deng-2 reservoirs in Hujiaba are 50 m thick, and 2.48% in average porosity and 0.49×10-3 μm2 in average permeability. In the region around Gucheng-Shuimo, Yuanba-Wangcang, and north of Tongjiang (Well MS1), mound-shoal complexes in restricted platform feature small single-layer thickness and cumulative thickness of 20-50 m. Due to dual effects of diagenesis and tectonization, fractured-porous and fractured-porous-cavity type reservoirs with moderate properties may form. For example, Deng-2 reservoirs in Shuimo are 47.2 m thick, and 2.18% in average porosity and 0.04×10-3 μm2 in average permeability.
4.2. Deng-4 Member
Deng-4 reservoirs in the ROI, turning up from the middle to the top of the member, feature small single-layer thickness (of 1-17 m), large number of layers, and large cumulative thickness (of 40-160 m) (Fig. 10). Most promising reservoirs are distributed widely in platform marginal mound-shoal complexes and inter-mound-shoal bottom lands in the region around Hujiaba-Guangyuan-Jian'ge-Langzhong. In spite of small single-complex thickness, these complexes have large cumulative thickness and extend in a large area (with continuous distribution of 10-40 km). Such complexes may turn into high-grade porous-cavity type or cavity-porous dolomite reservoirs after later burial-dissolution. For example, Deng-4 reservoirs in Hujiaba, Ningqiang, northern ROI are composed of thrombolitic dolostone, stromatolitic dolostone, and crystalline-grained dolostone with cumulative thickness of 165 m. With rich intra-stratal dissolved pores and cavities, these reservoirs have an average porosity of 5.377%, average permeability of 1.36×10-3 μm2, and surface porosity of over 10%. In the region around Gucheng-Shuimo, Yuanba-Wangcang, and Bazhong-Tongjiang, small intra-platform point mounds and shoals of 20-60 m thick may turn into porous and porous-cavity type reservoirs with moderate properties after later burial-dissolution. As per outcrop surveys, Deng-4 reservoirs in the area around Gucheng-Taoyuan-Shuimo are 40-60 m thick. The Deng-4 reservoirs in Gucheng have an average porosity of 5.17% and average permeability of 0.23×10-3 μm2. Those in Shuimo have an average porosity of 4.14% and average permeability of 0.13×10-3 μm2. This indicates that Deng-4 reservoirs in this area are generally of good quality.
In the region on the north of Guangyuan-Nanjiang, the Dengying Formation is uplifted above the land surface, losing hydrocarbon preservation conditions. But oil and gas may be preserved in deeply buried formations on the south. Therefore, the Deng-2 Member around Guangyuan-Jian'ge at the platform margin and the Deng-4 Member around Guangyuan-Langzhong are favorable exploration areas. Well CS1 drilled in Langzhong recently encountered mound-shoal reservoirs with dissolved pores and cavities from the middle to the top of the Deng-4 Member. Core observation shows the reservoir has surface porosity of 4-10% and the pores and cavities are partially filled with bitumen, proving huge exploration potential of this area.
5. Conclusions
Reservoir rocks in Dengying Formation of the ROI mainly include thrombolitic dolostone, stromatolitic dolostone, and crystalline powder-scale dolostone. There are also some dolomitic karst breccia reservoirs in the Deng-2 Member and dolarenite reservoirs in the Deng-4 Member. Deng-2 reservoirs are of low porosity and low permeability, in which the pore space is composed of intra-stratal isolated inter-algal frame dissolved cavities and residual cavities in grape lace- shaped cement partially filled with multi-phase dolosparite, with poor connectivity. No bitumen was found in the interstitial materials. Deng-4 reservoirs are of middle to low porosity and low permeability. Some reservoirs are of middle to high porosity and middle permeability, with pore space composed of intra-stratal inter-algal frame dissolved pores, dissolved cavities, intercrystalline pores, and intercrystalline dissolved pores partially filled with bitumen and some quartz, with moderate connectivity. Almost no dolomite was observed in the interstitial materials.
Dengying reservoir distribution and properties in the ROI are controlled by sedimentary facies and dissolution jointly. High-grade Deng-2 and Deng-4 reservoirs tend to appear in favorable sedimentary facies. Platform marginal mound-shoal complexes are the most promising facies, followed by intra-platform mound-shoal complexes in restricted platform. Deng-2 reservoirs tend to occur in favorable sedimentary facies coinciding with horizontal undercurrent zone formed by supergene karstification in Episode-I of the Tongwan tectonic movement, while Deng-4 reservoirs tend to occur in favorable sedimentary facies undergoing dissolution in the burial process.
Due to the differences in mound-shoal complex development and dissolution intensity, high-grade Deng-2 and Deng-4 reservoirs are likely to be around Ningqiang-Guangyuan- Jian'ge-Langzhong at the platform margin on the east side of the pull-apart trough. Some good reservoirs may also turn up in intra-platform mound-shoal complexes, for example around Gucheng-Guangwushan. It is suggested that Dengying hydrocarbon exploration may first focus on deeply buried reservoirs in the region around Guangyuan-Langzhong.
Reference
Formation conditions and exploration prospects of Sinian large gas fields, Sichuan Basin
DOI:10.1016/S1876-3804(13)60017-8
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Exploration potential of natural gas in the Sinian, Sichuan Basin, is evaluated based on research on the evolution of the Leshan-Longnüsi palaeouplift and reservoir forming conditions such as sedimentary facies, source rocks, reservoirs and caprocks. The Sinian has a potential to form large gas fields in the Sichuan Basin: (1) Large-scale inherited palaeo-uplift provided conditions for oil/gas formation and accumulation. (2) A stable sedimentary environment guaranteed the extensive development of source rocks and reservoirs. (3) Deng-2, Deng-3 and Deng-4 members overlap each other and are widely developed. (4) Multiple source rocks overlay each other vertically and are distributed widely, superimposed between reservoirs like “sandwiches”. (5) The Sinian has good preservation conditions, regional mudstone cap rocks are thick and faults are not developed. Considering these favorable reservoir forming conditions, four favorable prospective areas were selected: Leshan-Longnüsi Palaeouplift, Southeast Sichuan, East Sichuan, and Northwest Sichuan. Of them, the Leshan-Longnüsi palaeo-uplift is the most favorable prospect, where the litho-stratigraphic gas reservoirs in the slope deserve further exploration.
Accumulation conditions and enrichment patterns of natural gas in the Lower Cambrian Longwangmiao Fm reservoirs of the Leshan-Longnüsi Paleohigh, Sichuan Basin
DOI:10.1016/j.ngib.2014.10.006
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As several major new gas discoveries have been made recently in the Lower Cambrian Longwangmiao Fm reservoirs in the Leshan-Longn si Palaeohigh of the Sichuan Basin, a super-huge gas reservoir group with multiple gas pay zones vertically and cluster reservoirs laterally is unfolding in the east segment of the palaeohigh. Study shows that the large-scale enrichment and accumulation of natural gas benefits from the good reservoir-forming conditions, including: (1) multiple sets of source rocks vertically, among which, the high-quality Lower Paleozoic source rocks are widespread, and have a hydrocarbon kitchen at the structural high of the Palaeohigh, providing favorable conditions for gas accumulation near the source; (2) three sets of good-quality reservoirs, namely, the porous-vuggy dolomite reservoirs of mound-shoal facies in the 2nd and 4th members of the Sinian Dengying Fm as well as the porous dolomite reservoirs of arene-shoal facies in the Lower Cambrian Longwangmiao Fm, are thick and wide in distribution; (3) structural, lithological and compound traps developed in the setting of large nose-like uplift provide favorable space for hydrocarbon accumulation. It is concluded that the inheritance development of the Palaeohigh and its favorable timing configuration with source rock evolution are critical factors for the extensive enrichment of gas in the Lower Cambrian Longwangmiao Fm reservoirs. The structural high of the Palaeohigh is the favorable area for gas accumulation. The inherited structural, stratigraphic and lithological traps are the favorable sites for gas enrichment. The areas where present structures and ancient structures overlap are the sweet-spots of gas accumulation.
Formation, distribution, resource potential and discovery of the Sinian- Cambrian giant gas field, Sichuan Basin, SW China
DOI:10.1016/S1876-3804(14)60036-7
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The Anyue Sinian–Cambrian giant gas field was discovered in central paleo-uplift in the Sichuan Basin in 2013, which is a structural-lithological gas reservoir, with 779.9 km2 proven gas-bearing area and 4 403.8×108 m3 proven geological reserves in the Cambrian Longwangmiao Formation in Moxi Block, and the discovery implies it possesses trillion-cubic-meter reserves in the Sinian. Cambrian Formations in Sichuan Basin. The main understandings achieved are as follows: (1) Sinian–Cambrian sedimentary filling sequences and division evidence are redetermined; (2) During Late Sinian and Early Cambrian, “Deyang–Anyue” paleo-taphrogenic trough was successively developed and controlled the distribution of source rocks in the Lower-Cambrian, characterized by 20–160 m source rock thickness, TOC 1.7%–3.6% and Ro 2.0%–3.5%; (3) Carbonate edge platform occurred in the Sinian Dengying Formation, and carbonate gentle slope platform occurred in the Longwangmiao Formation, with large-scale grain beach near the synsedimentary paleo- uplift; (4) Two types of gas-bearing reservoir, i.e. carbonate fracture-vug type in the Sinian Dengying Formation and dolomite pore type in the Cambrian Longwangmiao Formation, and superposition transformation of penecontemporaneous dolomitization and supergene karst formed high porosity-permeability reservoirs, with 3%–4% porosity and (1–6)×10613 μm2 permeability in the Sinian Dengying Formation, and 4%–5% porosity and (1–5)×10613 μm2 permeability in the Cambrian Longwangmiao Formation; (5) Large paleo-oil pool occurred in the core of the paleo-uplift during late Hercynian—Indosinian, with over 5 000 km2 and (48–63)×108 t oil resources, and then in the Yanshanian period, in-situ crude oil cracked to generate gas and dispersive liquid hydrocarbons in deep slope cracked to generate gas, both of which provide sufficient gas for the giant gas field; (6) The formation and retention of the giant gas field is mainly controlled by paleo-taphrogenic trough, paleo-platform, paleo-oil pool cracking gas and paleo-uplift jointly; (7) Total gas resources of the Sinian–Cambrian giant gas field are preliminarily predicted to be about 5×1012 m3, and the paleo-uplift and its slope, southern Sichuan Basin depression and deep formations of the high and steep structure belt in east Sichuan, are key exploration plays. The discovery of deep Anyue Sinian–Cambrian giant primay oil-cracking gas field in the Sichuan Basin, is the first in global ancient strata exploration, which is of great inspiration for extension of oil & gas discoveries for global middle-deep formations from Lower Paleozoic to Middle–Upper Proterozoic strata.
Reservoir characteristics and control factors of Sinian Dengying Formation in central Sichuan
Controlling factors and distribution of reservoir development in Dengying Formation of paleo-uplift in central Sichuan Basin
DOI:10.7623/syxb201504003
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The most ancient deep Sinian carbonate rocks in paleo-uplift of central Sichuan Basin were taken as research object.Through the observation of macroscopic and microscopic reservoir characteristics and deep study on the controlling factor of reservoir development,the results show that Dengying Formation reservoir is classified as the composite genetic type of"bioherm-beach facies+karst";the development of reservoir is mainly controlled by bioherm-beach facies.The expanded or added pores(holes)due to karst superimposed reformation are the main body of current reservoir space.Meanwhile,it is put forward that the karst of Dengying Formation in Tongwan stage is stratabound-weathering crust karst(restrained by bioherm-beach facies)in early diagenesis stage,and the biohermbeach facies in paleo-uplift of central Sichuan Basin with extensively superimposition of regional karstification is the key for continuous development of the large-scale reservoir in Dengying Formation.Impacted by the differences in development degrees of bioherm-beach facies and the intensities of karst transformation,the degrees of reservoir development are diversified in various zones of the paleo-uplift,and the N-S trending belt zone on the west side of Gaoshiti-Moxi structure is the"sweet spot"of reservoir development.The key of reservoir prediction in Dengying Formation mainly depends on the effective identification of bioherm-beach facies and fine description of karst landform,so as to find the bioherm-beach facies similar to Moxi-Gaoshiti and the karst monadnock-slope superimposed area.
Origin and distribution of microbial dolomite reservoirs: A case study of 4th Member of Sinian Dengying Formation in the Sichuan Basin, SW China
DOI:10.1016/s1876-3804(17)30085-x
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Based on the drilling cores and slice observations,single well data and geochemical analysis,this paper analyzed features,origin and distribution of the 4~(th) Member reservoirs of Sinian Dengying Formation (Z_2dn_4) in the Sichuan Basin.It is demonstrated that the main reservoir is a set of microbial dolomites.The discovery of spherical dolomite has revealed that the dolomitization was related to the microbial action,belonging to the early protodolomite of low-temperature precipitation; the primary matrix pores and the penecontemporaneous eroded pores constituted the subject of the reservoir space,which was not due to the interlayer karst process related to the Tongwan Movement and burial-hydrothermal dissolutional process.The microbial mound-shoal complex and penecontemporaneous dissolution mainly control the development and distribution of the scaled reservoirs in Z_2dn_4.The microbial dolomite reservoir surrounding the intracratonic rift had a large thickness,good continuity and high quality,and was an important target of the survey.
Characteristics and controlling factors of microbial carbonate reservoirs in the Upper Sinian Dengying Formation in the Sichuan Basin, China
DOI:10.11743/ogg20170411
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Microbial carbonates dominate the 2ndand 4thmembers of the Dengying Formation in the Upper Sinian,Sichuan Basin. The 2nd member contains mainly thrombolites and grapestones with botryoidalis pores,microbe visceral pores,microbe framework pores,inter-clot dissolved pores as reservoir space. Vertically,there are three reservoir intervals with bitumen concentrated 117 m below the unconformity of the first stage of the Tongwan movement. The 4th member contains largely laminites and stromatolites,with fenestral pores and karst pores as the main reservoir spaces and inter-clot dissolved pores and microbe visceral pores as the minor reservoir spaces. Three reservoir intervals occur in the member with bitumen concentrated 134. 2 m below the unconformity of the second stage of the Tongwan movement. The development and distribution of microbial carbonate reservoirs in the members are suggested to be controlled mainly by shallow water tidal microbial mat,microbial structures,dolomitization and Mianyang-Changning intracratonic sag. The widespread mat might be the foundation of high-quality reservoirs,the microbial structures be the cause of the original reservoir differences,the dolomitization be the key to the formation of the reservoirs,and the weathering karst and burial dissolution controlled by the Mianyang-Changning intracratonic sag be the determinative factor for the modification and distribution of the reservoirs. And it is predicted that high-quality reservoirs in the Formation be along both sides of the margin area ofMianyang-Changning intracratonic sag.
Palaeogeography and tectonic- depositional environment evolution of the Late Sinian in Sichuan Basin and adjacent areas
Subdivision and correlation of the Sinian (Ediacaran) system in the Sichuan Basin and its adjacent area
DOI:10.1117/12.2195077
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Based on stratigraphic data from more than 10 outcrops sections and several dozens of boreholes,nine stratigraphic sub-regions have been recognized for the Sinian(Ediacaran)System of the Sichuan Basin and its adjacent area.They are:1)the Ya'an-Nanjiang stratigraphic sub-region,2)the Ebian-Yilong stratigraphic sub-region,3)the Weiyuan-Kaixian stratigraphic sub-region,4)the Yibin-Shizhu stratigraphic sub-region,5)the Leibo stratigraphic sub-region,6)the Longmenshan stratigraphic sub-region,7)the Chengkou-Wuxi stratigraphic subregion,8)the Tongzi-Enshi stratigraphic sub-region,and 9)the Yanhe-Dayong stratigraphic sub-region.Four of them are described in detail.The Sinian sequences are subdivided into the Doushantuo Formation/Labagang Formation and the Dengying Formation in ascending order.The latter includes four lithological members.A siliciclastic unit in the middle-upper Dengying Formation is considered as a key marker bed,which is considered as Member3 of the Dengying Formation.We propose that the Maidiping Member which yields early Cambrian small shelly animal fossils and was traditionally considered as the Member 4in the uppermost Dengying Formation,be removed from the Dengying Formation and be elevated to the Maidiping Formation.According to the latest chronostratigraphic chart of China,the base of the Sinian System is placed at the base of the Doushantuo cap dolostone,and the top of the Sinian System is represented by aparallel unconformity between the Dengying and Maidiping formations.The Doushantuo Formation corresponds to the Lower Sinian Series to the lower part of the Upper Sinian Series,including the Jiulongwan Stage,Chenjiayuanzi Stage and the Diaoyapo Stage.The Labagang Formation is only equivalent to the Diaoyapo Stage.The Dengying Formation corresponds to the Dengyingxia Stage of the upper part of the Upper Sinian Series.Stratigraphic correlation of the Sinian sequences of the Sichuan Basin and its adjacent area have been established based on lithostratigraphic,biostratigraphic,chemostratigraphic,geochronological,and well-log data.The sequence has also been correlated with the standard section of the Sinian System in the Yangzi Gorges area in Hubei Province.
Geological characteristics of the Sinian-Early Cambrian intracratonic rift, Sichuan Basin
DOI:10.3787/j.issn.1000-0976.2015.01.003
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The Mianzhu-Changning intracratonic rift was found in the western part of Gaoshiti-Moxi area in the middle Sichuan Basin and was closely related to the formation of the giant Sinian-Early Permian gas field in the Anyue Block.Based on the log and outcrop data of 27000 km seismic sections and the latest drilling data of Well Gao17,the boundary,distribution,deposition characteristics and evolution history of the Mianzhu-Changning intracratonic rift were systematically studied.The following findings were obtained.(1) This rift was mainly situated in the NS direction with a steep and stably-developed eastern boundary and a gentle western boundary developed at different periods and locations.(2) At the end of the second member of the Dengying Fm of the Sinian strata(Z_2dn_2),the rift,covering an area of 30000km~2,was characterized as an approximately symmetrical depression.At the end of Z_2dn_4,it was characterized as an 80000km~2 east-steep and west-gentle half graben controlled by faults in the east.(3) The sedimentary thickness of Z_2dn within the rift was thin,which consisted mainly of dark shales and micritic dolomites of shelf facies.On the contrary,the sedimentary thickness of Z_2dn on the eastern and western sides of the rift was thick with ultra thick carbonate rock,where platform margin reefs and grain banks were developed.(4) In_1m-_1 q(Early Cambrian Maidiping-Qiongzhusi period),black charcoal shale and politic siltstone of deep-water shelf facies were deposited;and microclasticrock-carbonate rock of hybrid shelf facies and relatively low-energy grain-beach dolomite were deposited in_1c and_1 l(Early Cambrian Canglangpu and Longwangmiao periods) respectively.(5) The evolution of the rift was primarily divided into four stages,i.e.,Z_2dn_1-Z_2dn_2 embryonic stage,Z_2dn_3-Z_2dn_4 developing stage,_1m-_1q filling and subsidence stage,and_1c-_1l shrinkage stage.In conclusion,both sides of this rift provide favorable conditions for gas accumulation and pooling in the Sinian-Early Cambrian strata in this study area and will become the main gas exploration target in the future.
Evolution characteristics of Central Sichuan palaeouplift and its relationship with Early Cambrian Mianyang-Changning intracratonic sag
DOI:10.3969/i.issn.1671-9727.2014.06.05
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In the light of the findings of the early Cambrian Mianyang-Changning intracratonic sag,this paper makes a study related to the structure and evolution for further knowing the basic geology in Sichuan Basin.Based on the palaeotectonic reconstruction of the top of the Sinian strata and by comparing the ranges of the intracratonic sag and the paleouplift at various stages,the present paper deciphers the relationship between the Mianyang-Changning intracratonic sag and the Central Sichuan paleouplift.The result shows that Sichuan Basin can be classified into 12 areas based on the evolutionary characteristics of the intracratonic sag and the paleouplift.The paper proposes that the Central Sichuan paleouplift formed with the evolution of the Cambrian intracratonic sag.The east side and west side of the intracratonic sag are independent tectonic regions,and only during Late Triassic period,they jointed a paleouplift.Their present structural features are the Southwest Sichuan uplift and the Central Sichuan uplift,respectively,especially resulting from the differential evolutions after the Himalayan period.Therefore,the Mianyang-Changning intracratonic sag controls the primary oil and gas geological conditions and the Central Sichuan paleouplift controls the formation,adjustment and preservation of the oil and gas reservoirs of the Lower Paleozoic-Sinian strata.
Lithofacies paleogeography and sedimentary model of Sinian Dengying Fm in the Sichuan Basin
DOI:10.3787/j.issn.1000-0976.2017.01.003
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For predicting the distribution of favorable reservoir facies belts of the super-large ancient Anyue carbonate gas field in the Sichuan Basin, through an analysis of structure and lithofacies palaogeography, the lithofacies palaeogeography and sedimentary model of the Sinian Dengying Fm was reconstructed based on the field outcrop, drilling and seismic data. As a result, achievements are made in four aspects. First, the basin and its periphery resided in an extensional tectonic setting in the Sinian. Intense extension led to the formation of the Deyang–Anyue intra-platform rift. Finally, the Sichuan Basin was divided into the palaeo-geographic pattern of "two uplifts and four sags". The "two uplifts" evolved into the platform, and the "four sags" evolved into the slope-basin environment. Second, in the depositional stage of the Deng 2 Member, some favorable reservoir belts developed, such as bioherm-shoal at the continental margin, bioherm-shoal at the rift margin, and bioherm-shoal in the platform. The bioherm-shoal at rift margin developed along both sides of the Deyang–Anyue rift, in a U-shape, with a width of about 5–40 km and a length of about 500 km. It connected with the platform margin belt at the continental margin to the west in the Shifang area, and to the north near Guangyuan area. Third, in the depositional stage of the Deng 4 Member, when the lithofacies palaeogeographic features in the Deng 2 Member remained, the platform margin belt at the rift margin evolved into two parts in the east and the west as a result of the continuous southward extensional faulting of the Deyang–Anyue rift until it finally crossed the basin from north to south. The eastern platform margin belt was located in the Guangyuan–Yanting–Anyue–Luzhou area, showing NS distribution with a length of about 450 km and a width of about 4–50 km. The western platform margin belt mainly developed in the Dujiangyan–Chengdu–Weiyuan–Yibin–Mabian area, showing an eastward arc distribution with a length of about 300 km and a width of 4–30 km. And fourth, the sedimentary model of rimmed platform with double platform margins in the Dengying Fm was established, providing a guidance for predicting the distribution of favorable reservoir facies belts.
Origin of the grape-like texture in rocks of Sinian age
The relations between paleokarst and reservoir porosity in Dengying Formation, Sinian of Ziyang and neighboring area, Sichuan
Geochemical characteristics and genesis of grapestone in Sinian Dengying Formation in south-eastern Sichuan Basin
DOI:10.1134/S1075701511030068
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For a long time,there is a dispute about the genesis of the grapestone in Dengying Formation.The geochemical characteristics of grapestone and basement rock are analyzed and compared in detail and the genetic environments of the grapestone are discussed in this article.Compared with basement rock,there are relatively obvious Ce negative anomaly and relatively low total content of rare-earth elements in the grapestone,which indicates that the grapestone and basement rock were not deposited simultaneously.There are low content of Na,K,high content of Mn,Fe,low content of Sr/Ba ratio,high content of 87Sr/87Sr ratio and both carbon and oxygen isotope negative anomaly,which illustrates that the grapestone was formed under fresh water.It can be corroborated that there was a brief exposure during the deposition of Dengying Formation which made Dengying Formation expose to earth's face,leached by meteoric water and emerge small-scale karst to create the conditions for the genesis of the grapestone.The brief exposure has very important significance for a better understanding of the genesis of the grapestone and the development of the reservoirs in the middle of Denying Formation.
Genesis and geological significance of upper Sinian Dengying dolostone with grape-lace shaped cement, Sichuan Basin
A kind of dolostone with "grape-lace shaped"cement is present in Upper Sinian Dengying Formation in Sichuan Basin. Because it is widely distributed in lateral but limited only in the middle-lower part of Member2 in longitudinal, it can be deemed as an identification mark for outcrops and rock cores. Based on macroscopic and microscopic characteristics in lithologic samples, this kind of rock, which is defined as "grape-lace shaped"dolostone, is comprised of three textures in microphotograph, i.e. surrounding rock, "grape-lace shaped" lamina and dolosparite. The origin of cement in this dolostone is analyzed and described.
Stratified dolomite reservoirs in Dengying Formation of Ziyang Region
Based on the macro, micro and geochemical characteristics of the cored intervals of Dengying Formation in Ziyang region and combined with routine petrophysical data, the basic features and the main controlling factors of the stratified dolomite reservoirs within the study area are studied and the spatial evolution and distribution pattern of porosity and permeability of this type of reservoirs are summarized in this paper. It is concluded that the stratified dolomite reservoirs are dominated by the strongly crystallized powder dolohate, fine dolomite, powder-fine residual.dolarenite and residual oolitic dolomite, and the pore space is predondnates by secondary solution vugs. Fractures, however, mainly play the role of connecting the pores except for possessing a certain capacity of storage. Fracture-vug and vug are the two main types of reservoir pore space. This kind of reservoirs are mainly located in the development areas of beach and calcareous mudnat facied, such as that along well Zi 3 and Well Zi 4 and their southwest area, which should be the main exploration targets where the upper and lower parts of the Deng 鈪 Member should be explored in order to discover the facies favourable for the development of good reservoirs.
Main control factors of pore development in deep marine carbonate reservoirs
DOI:10.1016/S1872-5813(07)60034-6
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Marine carbonate exploration is being flourishing in China,and most of these hydrocarbon fields belong to deep reservoirs.Oil and gas explorers have focused on how to find high quality reservoirs in deep carbonate layers.Compared with other countries,the deep carbonate layers in China formed earlier and experienced intense tectonic movement,and the type of diagenesis is more complicated,so exploration is more difficult.Many factors control the development of porosity in deep marine carbonate reservoirs: ①Favorable deposition environment is the base of the development of deep reservoirs;②Dolomitization has an important effect on the development of the pore of reservoirs;③Organic acid brought by hydrocarbon migration can produce dissolution pores during the deep burial period,faintly acid liquid hydrocarbon can repress diagenesis and save porosity of reservoirs;④Organic acid,CO_2 and other acidic gases generated by organic matter maturation can produce burial dissolution pores,among them H_2S and other acidic gases generated by TSR have key effect in expanding porosity and adding new dissolution pores;⑤Structural discordance and structure fault can produce huge dissolution,structure micro-crack reservoir permeability,and they form important migration pathways for corrosion fluids.
Mesogenetic dissolution of the middle Ordovician limestone in the Tahe oilfield of Tarim basin, NW China
DOI:10.1016/j.marpetgeo.2008.08.005
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The mesogenetic dissolution is well developed in the middle Ordovician Yijianfang formation (O 2yj) limestone, and the dissolution pores are very important for petroleum accumulation in the south slope area of the Tahe oilfield which lies in the north of the Tarim basin, northwestern China. Mottled, dotted or laminar dissolution can be observed in the O 2yj limestone. Under microscope, the grains, lime matrix and all stages of calcite cements (including oil-inclusion-bearing blocky calcite cements) can all be found dissolved ubiquitously. The stylolites in the limestone were enlarged and rounded because of dissolution. Some dolomite rhombs, precipitated along stylolites in burial environment, were found dissolved as well. The dissolution of the blocky calcite cements and dolomite rhombs and the enlarging of stylolites demonstrate that the dissolution took place in the mesogenetic environment. Concentration of trace elements, including REEs, of the eroded part of the O 2yj limestone is intermediate between that of the uneroded part and that of the underlying lower Ordovician limestone hydrocarbon source rocks. Both 13C PDB and 18O PDB values of the eroded part are less than those of the uneroded part, respectively. The geochemical characteristics indicate that the eroding fluids are hydrocarbon-bearing fluids coming from the underlying hydrocarbon source rocks.
Characteristics and significance of the burial dissolution of dolomite reservoirs: Taking the Lower Paleozoic in eastern Tarim Basin as an example
DOI:10.1016/S1876-3804(11)60045-1
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Based on the characteristics of core, thin-section, physical properties and carbon, oxygen and strontium isotope, the characteristics of Lower Paleozoic dolomite reservoirs, dissolution features and mechanism in eastern Tarim Basin were studied. Massive dissolution pore-cave dolomite is the main reservoir type in eastern Tarim Basin. The dissolution pores and caves are mainly medium to small with dense distribution. The fillings in dissolution pores and cracks commonly show negative 18O, generally far lower than the surrounding rock; slightly negative 13C, partially positive; slightly high 87Sr/ 86Sr. Proportion of CH 4 and CO 2 is high in fluid inclusions and some of them contain high proportion of H 2S, which demonstrated that the organic matter was at the high evolution stage when the inclusions were captured, suggesting deep burial circumstances. Silicification and high homogeneous temperature also indicate the presence of hydrothermal fluids. Dissolution includes organic acid dissolution and hydrothermal water dissolution. Acid fluid mainly includes organic acid, CO 2 and H 2S, which were mainly generated in the process of evolution of organic matter, with a little H 2S from sulfate reduction, a little CO 2 from volcanic activity. As the carrier of acid fluid, hydrothermal fluids overcome kinetic obstruction of dissolution and dissolve the deep dolomite. Major faults and associated faults provide channels for hydrothermal fluids, the acid fluid was further dispersed in dolomites through numerous inter-crystal pores caused by dolomitization and micro-cracks caused by compaction to complete dissolution for dolomite reservoirs.
Study progress on buried dissolution in carbonate rock
Constructive and destructive impacts are the main aspects of liquid effect on reservoirs.Karst cave and fracture derived from near-surface karst have been recognized as effective carbonate reservoir.There are different understandings among the academic circles on the topic which is the significant contribution of the buried dissolution in carbonate reservoirs to the secondary pore.On the basis of a large number of literatures being reviewed,this paper reviewed the classification of the mesogenetic fluid,summarized petrographic and geochemical marks,introduced the two opposite views and main control factors,emphasized the regional differences and the dependence of the channel system and improved the buried dissolution mode.Looking for main control factors of burial dissolution evolution in different region and considering to the dissolution and chocking effect the synchronously may be an effective way.The quantitative calculation of burial dissolution(or simulation) is an urgent problem in the field of secondary porosity of carbonate.
Scale and distribution of marine carbonate burial dissolutional pores
DOI:10.1016/j.jnggs.2016.08.003
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It is gradually accepted that porosity can be created in burial settings via dissolution by organic acid; TSR derived or hydrothermal fluids. The role of deep-buried carbonate reservoirs is becoming more and more important since the degree and difficulty in petroleum exploration of shallow strata are increasing. A profound understanding of the development scale and prediction of the deep-buried carbonate reservoirs is economically crucial. In addition to the formation mechanism, scale and distribution of burial dissolution pores in burial settings are focused on in recent studies. This paper is based on case studies of deep-buried (>4500 m) carbonate reservoirs from the Tarim Basin and Sichuan Basin. Case studies mentioned includes dissolution simulation experiments proposes that an open system is of crucial importance in the development of large-scale burial dissolution pores, the distribution pattern of which is controlled by lithology, pre-existing porosity, and pore throat structures. These findings provided the basis for evaluation and prediction of deep-buried carbonate reservoirs.
Overview of porosity evolution in carbonate reservoirs
Experimental simulation of dissolution and alteration of burial organic acid fluid on dolomite reservoir
DOI:10.3969/j.issn.1673-5005.2014.03.002
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In order to study the processes and effects of dolomite dissolution by organic acid under burial environment,the reactions of acetic acid( initial 0.2%) with sparry oolitic ash dolomite,sparry oolitic dolomite,and silty-fine dolomite were investigated using continuous flow diagenesis simulation system.The results show that the total amount of Ca2 + g2 +released from dolomite is( 13.19- 20.48) 10- 3mol /L.The dissolution quantities of dolomite reduce with the increase of burial depth.The weight lost of the samples on average is about 1%.The average porosity increases by 5%,and the average permeability rises as high as 300%.The results also indicate that the organic acid can still exist in deep burial environment,and it can also strongly dissolve dolomite.The internal pores and throats of dolomite are corroded to enlarge and interconnect,resulting in increased bulk volume of pores and throats but decreased total amount.Burial dissolution effect is significant,especially in permeability property.Inter-crystal and intra-crystal dissolution pores develop in dolomites,and the dolomites are eroded into honeycomb-like pores on the rock surface.The inter-crystal fractures of dolomite are enlarged by dissolution and become interconnected.The micro-corrosion characteristics of dolomite are beneficial to the accumulation and migration of oil and gas.
Research on reservoir-formed conditions of largemedium gas fields of Leshan-Longnvsi palaeohigh
Characteristics of source rocks, resource potential and exploration direction of Sinian and Cambrian in Sichuan Basin
DOI:10.11764/j.issn.1672-1926.2016.11.015
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The Anyue giant gasfield was discovered in the Sinian-Cambrian Central Sichuan region of the Sichuan Basin in 2013,with geological reserves up to 1 10~(12) m~3,which is the first time for the exploration of natural gas paleo-reservoirs in the world.The gas source studies suggest that the Sinian natural gas is originated from the Sinian and Cambrian hydrocarbon source rocks,and the systematical study on the Sinian and Cambrian ancient source rocks has important scientific and practical significance for the global oil and gas geologic domain of the ancient stratum.Based on the drilling data and field profile observation of Sinian and Cambrian in Sichuan Basin,with adoption of the interpretation data of 28 000km-seismic and new drilling data,combined with geochemical analysis of source rocks of 2 315 samples,this paper systematically studied the high quality hydrocarbon source rock center,where the source rocks are mainly distributed along the Mianzhu-Changning craton inner rift,with a accumulative thickness reaching 200-450 m,and 50-100 mfor the thickness of source rocks in other areas.The hydrocarbon source rocks of the Sinian-Cambrian contributed about 56%-63% of natural gas resources of the whole basin.Systematical evaluations have been conducted to the mudstone source rocks and their distribution in the -section of Sinian Dengying Formation,where TOCvalue is ranging from 0.04% to 4.73%,with an average of 0.65%.The thickness of the source rocks in central Sichuan region is ranging from 10 to 30m.The oldest Sinian source rocks that can form large gasfields in China were systematically studied for the first time,and the total gas production intensity of the Sinian source rocks in the great central Sichuan region is(15-28) 10~8 m~3/km~2,where the gas source conditions for the formation of large gasfields are available.By using the genetic method and analogy method,the amount of natural gas resources of the Sinian-Cambrian in the basin are re-evaluated as(4.65-5.58) 10~(12) m~3,and the resources potential of natural gas is huge.The amount of natural gas resources in the central Sichuan block accounts for about 66% of the total basin resources,which is the preferred selection for current exploration.
Formation conditions, accumulation models and exploration direction of large gas fields in Sinian-Cambrian, Sichuan Basin
DOI:10.11764/j.issn.1672-1926.2015.05.0785
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According to comprehensive research on forming conditions including sedimentary facies, reservoirs, source rocks, and palaeo-uplift evolution of Sinian-Cambrian in Sichuan Basin, it is concluded that: (1) large-scale inherited palaeo-uplifts, large-scale intracratonic rifts, three widely-distributed high-quality source rocks, four widely-distributed karst reservoirs, and oil pyrolysis gas were all favorable conditions for large-scale and high-abundance accumulation; (2) diverse accumulation models were developed in different areas of the palaeo-uplift. In the core area of the inherited palaeo-uplift, “in-situ” pyrolysis accumulation model of paleo-reservoir was developed. On the other hand, in the slope area, pyrolysis accumulation model of dispersed liquid hydrocarbon was developed in the late stage structural trap; (3) there were different exploration directions in various areas of the palaeo-uplift. Within the core area of the palaeo-uplift, we mainly searched for the inherited paleo-structural trap which was also the foundation of lithological-strigraphic gas reservoirs. In the slope areas, we mainly searched for the giant structural trap formed in the Himalayan Period.
Oil-cracking processes evidence from synthetic petroleum inclusions
DOI:10.1016/S0375-6742(03)00135-3
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A correlated study of petroleum inclusions synthesised in open quartz surfaces with the parent and the residual oil has beenperformed up to 350 C and 400 bar. Fourier transform infrared microspectrometry (FTIR), extracted fractions and GC-MS analysis have shown evidence of oil-cracking processes during the experiments. Results show that up to 250 C-212 bar, synthetic petroleum inclusions, the parent oil and the residual oil have very similar FTIR spectra and CH 2/CH 3 ratios. Although saturated and aromatic HC distributions of oils are similar, the asphaltene/resin ratio has been inverted. Such an inversion indicates that asphaltenes are cracked first during thermal maturation of oil. At 350 C and 400 bar, FTIR spectra and CH 2/CH 3 ratios are variable. Moreover, CO 2 and methane have been detected within the gas and the liquid phase of synthetic petroleum inclusions. Results show that the oil secondary cracking is effective at this temperature. The relative amount of aromatic nitrogen-, sulphur- or oxygen-containing heterocyclic compounds (NSO compounds) decreases. GC-MS analysis reveals that the cracking of the thermally most labile saturates and the dealkylation processes of aromatic hydrocarbons are more pronounced. Contemporaneous with these variations, the oil is enriched in saturates. Synthetic fluid inclusions act as a sampling of the oil at one p-T conditions and are thus the useful witnesses of the oil cracking during the experiments.
