Introduction
The Bohai Bay Basin is one of the major petroliferous basin in China, which have widespread hydrocarbon-rich sags and areas[1], and the Nanpu sag is one of them[2,3,4,5]. The latest resource potential evaluation shows that the Nanpu sag has petroleum resources of over 10 × 108 t[6]. In recent years, with the rise of exploration degree, the petroleum production from shallow formations (depth < 3500 m) has been decreasing constantly, the medium to deep formations (depth > 3500 m) become the main targets in the future exploration[7,8,9,10,11,12]. With the increase of depth, sandstone reservoirs would turn poorer in physical properties and even turn into tight sandstone reservoirs due to densification. Previous researches indicated that the limitation depth of the effective sandstone reservoirs in the Nanpu sag was about 4000 m[3]. But Well PG2 drilled in Nanpu No. 3 structural belt obtained an industry oil flow of 118 m3 oil and 11× 104 m3 gas a day (8 mm choke, natural production) from the first member of Shahejie Formation (Es1) (4248.0-4257.4 m), proving sandstone reservoirs more than 4000 m deep still have significant exploration potential. Therefore, the reservoirs in medium-deep formations believed to be ineffective before need to be studied further. The diagenetic process is one of the most important factor influencing the quality of clastic reservoirs[13,14]. We must get a clear understanding on the diagenetic characteristics of sandstone reservoirs to find out controlling factors and formation mechanisms of high-quality sandstone reservoirs of medium-deep depths, which will guide the petroleum exploration in the future. In this study, the first member of Shahejie Formation (Es1) deep sandstone reservoir (with a depth of over 4000 m on the whole) in the Nanpu No.3 structural belt and the third member of Shahejie Formation (Es3, with a depth of over 3500 m) sandstone reservoir in the Gaoshangpu Belt were compared; the characteristics and diagenetic features of the sandstone reservoirs were examined systematically by thin section analysis, scanning electron microscopy (SEM), X-ray diffraction analysis, and physical properties testing etc, the key controlling factors of and their effects on the high-quality sandstone reservoirs of different provenances in the Nanpu sag were analyzed by multiple regression, and the formation mechanisms of relatively high-quality sandstone reservoirs in the medium to deep depth background were discussed to provide references for petroleum exploration in deep formations.
1. Geological background
The Bohai Bay Basin is a lacustrine rift basin developed above the basement of the North China Craton with an area of approximately 2×105 km2. It has six sub-basins, the Nanpu sag is located in the north part of the Huanghua sub-basin, bounded by the Shaleitian Uplift in the south, the Xinanzhuang to Baigezhuang Fault in the north. The Nanpu sag is a sag with fault in the north and sediment overlap in the south, with an area of about 1932 km2[3,6,15-16]. There are eight structural belts in the Nanpu sag, and the study areas of this work are the Nanpu No.3 structural belt and Gaoshangpu structural belt (Fig. 1).
Fig. 1.
Fig. 1.
Structural map of the Nanpu sag (modified from reference [18]).
From the bottom to the top, the Tertiary sedimentary strata in the Nanpu sag include the Paleogene Shahejie (Es) and Dongying (Ed) Formations and the Neogene Guantao (Ng) and Minghuazhen (Nm) Formations[4,17] (Fig. 2). The sedimentary environment of the Shahejie Formation is shallow to deep lake and lacustrine delta; that of the Dongying Formation, shallow lake and lacustrine delta; and that of the Ng and Nm is braided river. The stratigraphic lithology of the study area is shown in Fig. 2.
Fig. 2.
Fig. 2.
Composite stratigraphic columnar section of the Nanpu sag (modifed from reference [18]).
The exploration practices show there are three sets of source rocks in the Nanpu sag, Es3, Es1, and Ed3, respectively[6,17]. The reservoirs include sandstone layers in the Paleogene fan and braided river deltas in Neogene fluvial facies[17]. The widely distributed mudstone and volcanic rock layers in Nm and Ng can act as regional seals.
2. The medium to deep sandstone reservoirs
2.1. Sedimentary environment and petrological characteristics
2.1.1. Sedimentary environment
The sedimentary system of the Es1 in the south part of the Nanpu sag is a braided delta sourced from the Shaleitian Uplift, the Nanpu No.3 structural belt has mainly delta front and predelta subfacies[12,18], which can be further divided into four types of microfacies: subaqueous distributary channel, interdistributary bay, mouth bar, and sheet sand. The subaqueous distributary channel has positive grain sequence of coarse sandstone at bottom and fine sandstone at top and takes on box shape and bell shape on spontaneous potential log curve. The mouth bar is characterized by an upward-coarsening grading, and appears as funnel shape on spontaneous potential log. The sheet sand with small thickness appears as finger shape on spontaneous potential log curve. The interdistributary bay is mainly composed of dark mudstone and fine sandstone, and takes on flat-tooth shape on spontaneous potential log curve. The sandstone layers of subaqueous distributary channel and mouth bar facies are the major petroleum reser-voirs in the study area. The sedimentary system of the Es3 in the north part of the Nanpu sag is a fan delta sourced from the Baigezhuang Uplift[19], and the study area has largely fan delta front and predelta subfacies, and subaqueous distributary channel, interdistributary bay, and mouth bar microfacies. The sandstone layers of subaqueous distributary channel and mouth bar facies take the majority of the reservoirs in the study area (Fig. 3).
Fig. 3.
2.1.2. Petrological characteristics
According to the classification scheme of Folk[20], the Es1 sandstone reservoirs in Nanpu3 structural belt are primarily lithic arkose, followed by arkose and feldspathic litharenite (Fig. 4). The rock fragments are composed of mainly metamorphic rock fragments, and minor sedimentary and volcanic rock fragments, with higher compositional maturity. The sandstones are mostly sandy conglomerate, anisometric medium to coarse- grained sandstone, with high sorting coefficient of 1.96 on average, medium grain size from 0.078 to 3.100 mm, 0.890 mm on average, subangular to subrounded grains, and moderate textural maturity (Table 1).
Fig. 4.
Fig. 4.
Triangular diagram showing the detrital grain composition of the Es sandstone samples in the Nanpu sag. Q—quartz, F—feldspar, L—rock fragment.
Table 1 Physical properties of Shahejie Formation reservoirs in the Nanpu sag.
| Structural belt | Formation | Lithology | Average porosity/% | Permeability/ 10-3 μm2 | Reservoir evaluation | Pore types | Plane porosity/% |
|---|---|---|---|---|---|---|---|
| Gaoshangpu | Es3 | Feldspathic litharenite and lithic arkose | 9.8 | 0.48 | Tight reservoir | Mainly secondary pores | 6.2 |
| Nanpu No.3 | Es1 | Lithic arkose | 13.4 | 170.33 | Low porosity but medium- high permeability | Mainly primary pores | 4.6 |
The Es3 sandstone reservoirs in the Gaoshangpu structural belt are mainly feldspathic litharenite and lithic arkose (Fig. 4). The rock fragments with higher content consist mainly of metamorphic and volcanic rock fragments, and few sedimentary rock fragments. The feldspar primarily consists of plagioclase and K-feldspar, with lower compositional maturity. The reservoirs have an average sorting coefficient of 1.52, and median grain size from 0.09 to 0.27 mm, 0.17 mm on average; subangular to subrounded grains, moderate textural maturity (Table 1).
2.2. The sandstone reservoirs
2.2.1. Physical properties
The reservoir of the Es1 in Nanpu No.3 structural belt is of low porosity but medium to high permeability, and the reservoir of the Es3 in the Gaoshangpu structural belt is tight sandstone reservoir (Fig. 5). Both Es1 and Es3 sandstones have good correlation between porosity and permeability. Overall, the Es1 reservoir in Nanpu structural belt has better physical properties than the Es3 reservoir in Gaoshangpu structural belt, especially higher permeability. Some samples of the Es1 even with a porosity of less than 10% still have relatively high permeability (> 100×10-3 μm2) (Table 1).
Fig. 5.
Fig. 5.
Scatter diagram of the relationship between porosity and permeability of the Shahejie Formation in the Nanpu sag.
2.2.2. Pore types
Based on thin section observation, the Es sandstone samples in the Nanpu sag mainly have two types of storage space, primary pore and secondary pore. After reformation by diagenetic processes, the primary pores are irregular in shape (Fig. 6a-6d), the secondary pores, irregular in shape, are formed by dissolution of feldspar and rock fragments (Fig. 6a-6e). Meanwhile, the reservoirs have some microfractures (Fig. 6d, 6f), which have little contribution to porosity, but can effectively improve the permeability of the reservoirs[3]. Compared with the Es3 reservoir in Gaoshangpu structural belt, the Es1 reservoir in Nanpu3 structural belt has more microfractures, and this may be one of the reasons why Es1 reservoir has low porosity but moderate to high permeability.
Fig. 6.
Fig. 6.
Pore types of Shahejie Formation reservoirs in the Nanpu sag. (a) Primary pore, secondary intragranular dissolution pores (Well G94-4, 3 671.09 m); (b) Secondary intergranular dissolution pore (Well GJ3102-8, 3 499.95 m); (c) Secondary intergranular dissolution pore (Well G66X3, 4077.50 m); (d) Primary pore, secondary intergranular dissolution pore, secondary intragranular dissolution pores and microfractures (Well PG2, 4252.50 m); (e) Secondary intergranular dissolution pore (Well PG2, 4 251.90 m); (f) Microfractures (Well NP306X3, 4 987.51 m); (g) Primary pore (Well G66X3, 4 074.89 m); (h) Intergranular dissolution pore and primary pore (Well PG2, 4 251.90 m); (i) Intragranular dissolution pores (Well G8X1, 3 830.98 m).
The Es1 reservoir has largely primary pores, and a thin section porosity range from trace level (< 1%) to 11.2%, 4.6% on average, in which thin section porosity of primary pores is from trace level to 4.7%, and 2.6 on average; and the thin section porosity of secondary pores is from trace level to 8.2%, and 2% on average (the number of samples is 67). In comparison, sandstone samples from the Es3 in Gaoshangpu have mainly secondary pores, and thin section porosity from trace level to 9.5%, 6.2% on average. The primary pores contribute a thin section porosity from trace level to 2.7%, 1.7% on average, while secondary pores contribute a thin section porosity from trace level to 7.2%, 4.5% on average (the number of samples is 31) (Table 1).
2.3. The diagenetic processes
As the Es Formation in general is large in buried depth (Es3 > 3500 m; Es1 > 4000 m), the formation has experienced strong compaction, the grains in the formation are mainly in linear contact, a small number in stylolite contact (Fig. 7a, 7b).
Fig. 7.
Fig. 7.
Diagenetic features of the Shahejie Formation reservoir in the Nanpu sag. (a) Grain stylolite contact (Well G23X8, 4 259.00 m); (b) Grain point contact, grain line contact, grain concave-convex contact (Well PG2, 4 252.09 m); (c) Quartz overgrowth (Well NP306X3, 4 981.26 m); (d) Quartz overgrowth (Well G66X3, 4 074.67 m); (e) Calcite cement (Well G66X3, 4 074.20 m); (f) Fe-calcite cement (Well PG2, 4 251.44 m); (g) Feldspar dissolution (Well PG2, 4 254.95 m); (h) Feldspar dissolution (Well GJ3102-8, 3 490.76 m); (i) Rock fragment dissolution (Well GJ3102-8, 3 492.50 m).
The most common cements in the Es sandstone are carbonate cements, quartz overgrowth, and clay minerals. The carbonate cements are dominated by calcite (Fig. 7e), which appears as pore-filling and replacing cement. The Es1 sandstone samples have carbonate cements contents from trace level to 23.2%, 5.9% on average, and Fe-calcite in local parts (Fig. 7f). The Es3 sandstone samples have carbonate cements contents from trace level to 27.4%, 3.3% on average, which appear in forms of microcrystalline and pore-filling.
Quartz overgrowth is a major cause of porosity reduction of reservoirs in many petroliferous basins[21,22]. The Es1 sandstone samples have quart overgrowth contents from trace level to 4.3%, and 0.9% on average. Es3 sandstone samples have quart cement contents from trace level to 4.3%, 2.0% on average. The quart cements come in microquartz and quartz overgrowth (Fig. 7c, 7d), the quartz overgrowth is commonly developed, filling most intergranular pores (Fig. 8a).
Fig. 8.
Fig. 8.
Types of clay minerals in Shahejie Formation reservoirs, the Nanpu sag. (a) Microlitic quartz and Illite (Well PG2, 4 248.98 m); (b) Mixed-layer illite/smectite (Well PG2, 4 251.21 m); (c) Illite (Well PG2, 4 251.78 m); (d) Chlorite (Well NP306X1, 4 240.21 m); (e) Kaolinite (Well NP306X6, 4 413.56 m); (f) Chlorite (Well G66X3, 4 074.20 m); (g) Kaolinite (Well G8X1, 3 813.15 m); (h) Mixed-layer illite/smectite (Well G8X1, 3 813.85 m); (i) Illite (Well G8X1, 3 827.15 m). I—Illite; I/S—Mixed-layer illite/smectite; Ch—Chlorite; K—Kaolinite; MQ—Microlitic quartz; F—Feldspar.
The clay minerals in Es sandstone often appear as grain coatings and filling of intergranular pores. The Es1 has a wide range of clay minerals contents from 2% to 15.7%, with an average of 7.8%. The mixed-layer illite/smectite (I/S) and illite are the main types in the clay minerals (Fig. 8a-8c), the former has a relative content of 61%-96% (on avg. 77.8%), and the latter 3.2%-33.9% (on avg. 13.9%). The sandstone samples have low chlorite contents from trace level to 21%, on average 5.6% (Fig. 8d); lower kaolinite contents from trace level to 9%, 2.7% on average. In the north part, the Es1 differ widely from Es3 in clay minerals, with contents from 6.4% to 15.5% (on avg. 11.8%). I/S and kaolinite take the majority of clay minerals (Fig. 8g, 8h), accounting for 58.4% and 19.2% on average, respectively. Chlorite and illite follow behind (Fig. 8f, 8i), accounting for 12.5% and 9.0% on average respectively.
Dissolution is a key mechanism in forming high-quality reservoirs in deep depth[23,24,25,26]. The main dissolution minerals in the study area are feldspar and rock fragment (Fig. 7g-7i), in which, feldspar dissolution takes dominance. The feldspar grains are dissolved along edges and cleavages, forming irregular intragranular dissolution pores. The dissolution process is commonly seen in both Es1 and Es3 reservoirs, and the Es3 reservoir has more significant and common dissolution than Es1.
2.4. Diagenetic evolution sequence
According to diagenetic stage classification criterion[27] and the Chinese standard classification scheme of sandstone diagenesis “the division of diagenetic stages of the clastic rocks” (SY/T5477-2003), based on the boundary conditions including terrestrial heat values and so on of the Bohai Bay Basin[28], the diagenetic sequence of the study area has been reconstructed[22,29-30]. The reservoirs in the study area have experienced eodiagenesis (at the depth of <2 km, temperature of<70 °C, and Ro< 0.5%) and mesodiagenesis (at the depth of >2 km, temperature of >70 °C, and Ro >0.5%).
The Es1 sandstone in Nanpu3 structural belt mainly experienced mechanical compaction and cementation of calcite and quartz overgrowths in the eodiagenesis stage. The mechanical compaction is the main diagenetic process causing porosity reduction. As the overburden pressure increases during burial, the grains in the sediments rearrange and change in contact relationship under the effect of mechanical compaction[24,31]. In the eodiagenesis stage, the rock is in a weakly consolidated and semi-consolidated state, compaction has a significant effect on reservoir physical properties. Meanwhile, calcite and quartz overgrowth precipitate as cements. Some researchers hold the view that the meteoric water plays an important role in early diagenesis stage[32]. But the Es1, which deposited in lacustrine and lake delta[3,25], rarely exposed to the meteoric water. Therefore, only a small amount of feldspar and rock fragment might be dissolved by meteoric water, instead, the main source of the dissolution fluid should be the organic acid formed by the maturation of the source rock[12,18]. In the mesodiagenesis stage, the diagenetic processes mainly included: strong compaction leading to further deterioration of physical properties; further development of quartz overgrowth; enrichment of kaolinite and illite; and dissolution of feldspar. The cementation of quartz overgrowth, and formation of illite, and I/S made the reservoir physical properties turn worse further. On the contrary, the dissolution of feldspar produced many secondary pores, which provided storage space for hydrocarbon. Compaction made the porosity of sandstone reduce further and the ductile grains deform, and the grains turn into line and stylolite contact. With the increase of depth and temperature, the CO2 and organic acid formed by the maturation of source rock dissolved feldspar and calcite. As mentioned before, Es1 sandstone has higher feldspar content, so a large number of secondary pores were produced, and the dissolution of feldspar is the primary mechanism of the formation of secondary pores. Then the Es1 reservoir entered the mesodiagenesis A2 stage, due to the decrease of organic acid concentration, late carbonate cements (mainly Fe-calcite) were formed (Fig. 9); the dissolution weakened, part of the secondary dissolution pores were destroyed by cementation an compaction, at the same time, the kaolinite formed by the dissolution of feldspar transformed to illite under high temperature, which is the reason of the enrichment of illite in the Es1 sandstone[22,23,24,25,26,27,28,29,30,31,32,33].
Fig. 9.
The Es3 reservoir in Gaoshangpu structural belt is similar to Es1 reservoir in diagenetic evolution, but with shallower buried depth, the Es3 is in mesodiagenesis A1 stage, and the secondary pores formed by dissolution of feldspar and rock fragment in it haven’t been severely destroyed by compaction and cementation, this also explains the abundant secondary pores in Es3 sandstone in the north. Meanwhile, due to low temperature at shallower depth, kaolinite in Es3 hasn’t totally transformed into illite, and the late carbonate cements haven’t appeared (Fig. 9).
3. Controlling factors and formation mechanisms of medium to deep high-quality sandstone reservoirs
In general, with the increase of depth, the reservoirs become worse in physical properties gradually, so most medium-deep reservoirs are much worse in physical properties than shallow reservoirs in the same area. However, under some conditions, medium-deep formations still have relatively high-quality reservoirs[12,18], and the development of these reservoirs is one of the controlling factor determining the resource potential of petroleum system in medium-deep formations[18]. The formation mechanisms of medium-deep high-quality reservoirs are complicated. According to previous studies, the formation mechanisms mainly include: original sedimentary conditions[12]; deep dissolution process[18,26]; abnormal overpressure[25]; early hydrocarbon charging[25]; configuration of sandstone and mudstone[18,23]; and existence of structural fractures[22-23,26]. Relatively high-quality deep reservoirs in different areas have different formation mechanisms. In the same area, different factors may work together on the same target layer. Therefore, based on analysis of traditional key controlling factors, the effects of different factors on reservoirs in the study area were described in detail by using mathematical geology method to reveal the formation mechanisms and predict the favorable exploration area of medium-deep reservoirs in the Nanpu sag.
3.1. Controlling factors of high-quality reservoirs
3.1.1. The effects of sedimentary environment and rock composition and fabric
The favorable sedimentary environment is conducive to the development of sandstone. Taking the Es33 sub-member as an example, its microfacies and sandstone planar distribution are shown in the Fig. 10. It can be seen that the subaqueous distributary channels are mainly concentrated in the east and northeast of the Gaoshangpu structural belt, the mouth bars are also widespread, the sheet sand is limited in distribution, and the interdistributary bay sands are in the south and west of the structural belt. The sandstone thickness has a good correlation with the planar distribution of the sedimentary microfacies. The sedimentary environment can not only influence the scale of sandstone, but also control the rock fabric (grain size and sorting) of the reservoir, thus affecting the original physical properties of the reservoir[22,26-27,34]. Compared with the low-energy environment (interdistributary bay), the sediments in the high-hydrodynamic sedimentary environment (subaqueous distributary channel) have coarse grain size and better compaction resistance[14,26]. According to the statistical results of physical properties of the Es1 sandstones of different sedimentary microfacies (grain size, porosity and permeability), the subaqueous distributary channel sandstone has the coarsest grain size, with a median grain size of 1.06 mm, and good sorting. The sandstone has a porosity from 12% to 16%, 13.1% on average, and permeability from 40 to 640×10-3 μm2, 60.4×10-3 μm2 on average. Many primary intergranular pores can be observed in thin sections of this sandstone (Fig. 11), which indicates the reservoir has good physical properties. Compared with subaqueous distributary channel sandstone, the mouth bar and sheet sandstones have smaller grain sizes and poorer physical properties, and thin sections also show the sandstones have hardly any pores (Fig. 11). The Es3 tight sandstones show the same regularities. The subaqueous distributary channel, mouth bar, and sheet sandstones have porosities of 13.4%, 10.1%, and 9.2%, permeabilities of 2.52× 10-3 μm2, 0.74×10-3 μm2, and 1.1×10-3 μm2, and median grain sizes of 0.21 mm; 0.19 mm and 0.17 mm respectively.
Fig. 10.
Fig. 10.
Sedimentary microfacies (a) and sandstone thickness contour (b) of the Es33 in the Gaoshangpu structural belt of the Nanpu sag.
Fig. 11.
Fig. 11.
Reservoir properties and sedimentary microfacies of the Es1 reservoir in the Nanpu No. 3 structural belt[18].
3.1.2. The effects of diagenetic processes
3.1.2.1. The effect of compaction
Based on the intergranular volume and the cement content, we can further calculate the compaction and cementation rate, and the relative contribution of two diagenesis to reservoir porosity can be determined according to compaction and cementation rate. Our results show that the reservoirs in the study area have compaction rates from 53.5% to 95.1%, 69.1% on average, and cementation rates from trace level to 31.2%, 7.7% on average. Therefore, compaction is the primary mechanism of porosity decrease, and compaction resistance ability is the prerequisite for development of high-quality reservoir.
To some extent, the original components of reservoir rock determine the compaction resistance ability of the reservoir. Generally, the rock with higher content of rigid particles (quartz and metamorphic rock fragment) has higher compaction resistance, which is conducive to the preservation of primary pores[12,25]. From statistics of rigid grain content and compaction rate (Table 2), with higher rigid particle content, the Es1 reservoir has lower compaction rate than the Es3 reservoir, so the Es1 reservoir still has many primary pores, but the Es3 reservoir has mainly secondary pores. The Es1 sandstone reservoir in Nanpu3 structural belt has parent rocks of Archean granite[3], so this reservoir has higher quartz and metamorphic rock fragment content than the Es3 reservoir (Es1: 52.4%; Es3: 43.8%), and higher compaction resistance, which contributes greatly to the development of medium- deep high- quality reservoir.
Table 2 Statistical results of diagenesis and petrology characteristics of the Shahejie Formation reservoirs in the Nanpu sag.
| Formation | Rigid particle content/% | Compaction rate/% | Feldspar content/% | Dissolution rate/% | ||||
|---|---|---|---|---|---|---|---|---|
| Range | Average value | Range | Average value | Range | Average value | Range | Average value | |
| Es1 | 44-66 | 52.4 | 53.5-78.6 | 67.7 | 31-52 | 39.4 | 6.8-33.7 | 19.3 |
| Es3 | 28-48 | 43.8 | 63.9-95.1 | 72.6 | 20-39 | 31.9 | 13.0-35.7 | 25.9 |
3.1.2.2. The effects of cementation
Cementation is an important factor influencing reservoir quality in certain cases[22-23, 26, 34]. The Es sandstones are affected by the cementation at different degrees. Carbonate cement is the main cement in the study area, there are two stages of carbonate cements identified in Es1 reservoir, but the Es3 reservoir only has one stage of carbonate cement. The first stage in Es3 of cement is mainly sourced from the formation water, in some samples, calcite poikilitic cementation can be observed (Fig. 7e), the early cementation prohibited the later compaction to some extent, and the carbonate cements filling intergranular pores and blocking throats have negative impact on physical properties of the reservoir. The second stage of carbonate cement can be identified in the Es1 sandstone. With the diagenetic evolution, the organic acid was consumed in feldspar dissolution, the formation water changed into alkalinity, and the late stage of carbonate cement precipitated in the secondary pores formed by feldspar and early carbonate cement dissolution. Since the late stage of carbonate cement appeared later than the charge of large amount of organic acid (Fig. 9), therefore, this kind of cement was hardly dissolved by organic acid[25], impairing physical properties of reservoir.
3.1.2.3. The effects of dissolution
Dissolution has major positive effect on the formation of medium-deep high-quality reservoirs[12,18,22-23,26,35]. At the same time of the development of Es1 and Es3 sandstone reservoirs, the two sets of major source rocks in the Nanpu sag[5,36-37] matured and released massive organic acid, resulting in large-scale dissolution. The sediments formed in strong hydrodynamic sedimentary environment (subaqueous distributary channel) had better original physical properties and pore connectivity which was conducive to the flow of organic acid, providing convenient conditions for dissolution[34]. Kaolinite is a kind of mineral associated with feldspar dissolution, so it can reflect the degree of feldspar dissolution. The Es3 reservoir has higher kaolinite content (19.2% on average), which indicates stronger dissolution. According to the statistics of diagenetic and petrologic characteristics, dissolution has played an important constructive role in the formation of the Es1 and Es3 reservoirs, especially in the Es3 reservoir. The Es3 reservoir has much lower feldspar content and much higher dissolution rate than Es1 reservoir (Table 2). The reason is the Es3 is the set of source rock with the strongest hydrocarbon generation capacity in the Nanpu sag[5], during the thermal evolution, the source rock generated a large amount of organic acids, and the feldspar in the reservoir provided material basement for dissolution. The relationship between feldspar content and reservoir dissolution rate in the study area shows that the enrichment of feldspar promotes the development of secondary pores (Fig. 12). Meanwhile, the Es3 reservoir has higher slope and correlation coefficient of the fitted formula than the Es1 reservoir, which indicates feldspar dissolution has more constructive effect on the Es3 reservoir. Thin sections show many intergranular and intragranular dissolution pores (Fig. 7h) and higher plane porosity of secondary dissolution pore (on average 4.5%), which shows the Es3 reservoir has suffered strong dissolution. Therefore, the dissolution has had stronger improvement to the Es3 reservoir in Gaoshangpu than the Es1 reservoir in Nanpu.
Fig. 12.
Fig. 12.
Scatter diagram of the relationship between dissolution rate and feldspar content of the Shahejie Formation in the Nanpu sag.
3.2. Quantified characterization of the factors affecting high-quality reservoir formation
The physical properties of reservoir are controlled by many factors. The sedimentary environment and properties of parent rock determine the grain size, sorting, roundness and composition of the reservoir[12,24]. Diagenetic processes rework the reservoir on the basis of sedimentation[12,18,22,26]. These two factors work together to affect the reservoir quality. At present, correlation analysis between single factor and reservoir physical property is widely used to find out the main factors controlling reservoir, this method can reflect the influence of certain factors to reservoir physical property in some certain extents, but can’t quantify the relative magnitudes of the factors, so it cannot accurately reveal the formation mechanisms of high-quality reservoir. In this study, multivariable regression analysis was used to analyze the effects of different factors on reservoir. The relationships between controlling factors mentioned above, including sedimentary factors (the content of quartz, feldspar, rigid particle, and soft rock fragment; grain size, and sorting coefficient) and diagenetic factors (compaction rate, cementation rate, and dissolution rate) and porosity were fitted by multiple linear regression function of SPSS software based on the measured data of Es1 and Es3 reservoirs (Table 3). The positivity and negativity of the coefficients before the parameters represent the constructive and destructive factors respectively, and the value of the parameter indicates the magnitude of the factor impact, the larger the value of the factor, the more significant the impact of the factor to the reservoir porosity is. In this way, we have quantified the effects of different controlling factors to further reveal the formation mechanisms of relatively high-quality reservoir.
Table 3 Multivariable regression analysis results of factors controlling the Shahejie Formation reservoir physical properties in the Nanpu sag.
| Formation | Multivariable regression analysis equation | Correlation coefficient | Constructive factor sequence | Destructive factor sequence |
|---|---|---|---|---|
| Es1 | ϕ=0.525Q+0.309F+0.262H-0.025S+4.548M-1.234SC-3.588P-2.859C+1.673D-21.214 | 0.452 | M>D>Q>F>H | P>C>SC>S |
| Es3 | ϕ=0.127Q+0.170F+0.067H-0.012S+9.666M-0.015SC-3.415P-0.835C+27.771D-3.719 | 0.396 | D>M>F>Q>H | P>C>SC>S |
In the multiple regression equation of porosity, the effects of the influencing factors are reflected by the corresponding parameters in the equation. For the Es1 reservoir, the constructive factors in descending order are: grain size, dissolution rate, quartz content, feldspar content, and rigid particle content. The destructive factors in descending order are: compaction rate, cementation rate, sorting coefficient and soft rock fragment content. The correlation coefficient of the multiple regression equation is 0.452. For the Es3 reservoir, the constructive factors in descending order are: dissolution rate, grain size, feldspar content, quartz content, and rigid particle content; the destructive factors in descending order are: compaction rate, cementation rate, sorting coefficient and soft rock fragment content, and the correlation coefficient of this equation is 0.396.
3.3. Formation of Shahejie Formation high-quality reservoirs in the Nanpu sag
According to the controlling factors and their quantitative characterization results mentioned above, the formation mechanisms of Es1 and Es3 reservoirs were examined.
The Es1 reservoir in Nanpu 3 structural belt, better in quality, is low porosity but medium to high permeability reservoir (with an average porosity of 13.4%, and average permeability of 170.33×10-3 μm2) and has largely primary pores. The sedimentary environment has significant control on the reservoir quality. The high-quality reservoir mainly occurs in the subaqueous distributary channel microfacies. The sediments in high hydrodynamic environment have coarser grain size and higher compaction resistance, therefore, the primary pores in the sediments can be well preserved. In contrast, the reservoirs depositing in low energy sedimentary environments (mouth bar and sheet sand) have smaller median granularity and lower compaction resistance, thus the reservoirs have poorer physical properties. Meanwhile, the rigid particles provided by the parent rock sustain the compaction resistance ability of the reservoir, while the later dissolution further improves the reservoir's quality. Therefore, favorable sedimentary microfacies, dissolution process, and high content of rigid particles jointly control the development of high-quality reservoir of the Es1[12,18]. According to the quantitative formula of reservoir porosity obtained above, grain size and quartz content have greater influence on reservoir porosity than dissolution rate and feldspar content. Therefore, the strong compaction resistance ability provided by coarse grain size and high rigid particle content (quartz and rigid rock fragment) is more important than the dissolution process for the formation Es1 reservoir. The genetic mechanism of the high-quality Es1 reservoir can be summarized as follows: the pore preservation by compaction resistance takes dominance, while dissolution is auxiliary.
The Es3 reservoir is tight sandstone (with an average porosity of 9.79%, and permeability of 0.48 ×10-3 μm2), which has largely secondary pores. The sedimentary environment has some influence to the reservoir quality. As the main storage space in the reservoir is secondary dissolution pore, so the local favorable exploration area of the tight reservoir is mainly controlled by the intensity of dissolution. At the same time, from the quantitative formula of the reservoir porosity, it can be seen that dissolution rate and feldspar content have significant control on reservoir porosity, indicating that dissolution is more important for improving porosity than the ability resisting compaction for the Es3 reservoir. Clearly, dissolution is the most important mechanism and sedimentary environment and compaction resistance ability provided by rigid particle is the secondary mechanism for the formation of favorable exploration areas of tight reservoirs[19].
Due to the differences in formation mechanisms, the reservoirs differ widely in physical properties. Compared with the Es3 reservoir in Gaoshangpu, the Es1 reservoir in Nanpu3 structural belt has higher diagenetic evolution degree, and as some secondary dissolution pores are destroyed by later compaction and cementation, its porosity of secondary pores is much lower than that of the Es3 reservoir. The Es1 high-quality reservoir in Nanpu3 structural belt is mainly controlled by the strong compaction resistance ability provided by the original sedimentation, and more primary pores in the reservoir have been preserved during burial. Compared with secondary pores, primary pores have better connectivity[38,39,40], in addition, the reservoir has microfractures, so the reservoir is characterized by medium to high permeability. In contrast, the favorable exploration area of Es3 reservoir is mainly controlled by the dissolution process, although the reservoir is lower in diagenetic evolution stage and many secondary pores have been preserved, the secondary pores are poorer in connectivity and the reservoir is smaller in grain size and lower in rigid particle content (quartz and metamorphic rock fragment)[43,18], therefore, the reservoir has poorer compaction resistance ability and few microfractures, so it has poorer quality than the Es1 sandstone in Nanpu structural belt.
3.4. Favorable exploration area of the Shahejie Formation in the Nanpu sag
Due to the different formation mechanisms of relatively high-quality reservoirs in different structural belts, the principles to predict favorable exploration areas are also different. For the Es1 reservoir in Nanpu3 structural belt, as the sedimentary environment is the main controlling factor for the formation of high-quality reservoir, when predicting its favorable exploration areas, we selected the most favorable sedimentary environment as the future exploration area. In this study, taking Es11 sub-member as an example, the subaqueous distributary channel was selected as the first class favorable exploration area (the most favorable exploration area), the mouth bar and sheet sand were selected as the second class favorable exploration areas (the favorable exploration area), and the interdistributary bay was selected as the non-favorable exploration area (Fig. 13a). In contrast, for the Es3 reservoir, although the sedimentary environment has a certain control effect on the high-quality reservoir, it is mainly controlled by dissolution intensity, and strong dissolution creates local favorable exploration areas in the tight reservoir. Therefore, in predicting its favorable exploration areas, on the basis of sedimentary environment, the dissolution intensity should be taken into consideration. Since the dissolution of the Es3 sandstone is mainly from the organic acids generated by the source rock, the hydrocarbon expulsion intensity of the source rock was taken as the parameter to measure the dissolution intensity. According to the hydrocarbon expulsion intensity of the source rock and the sedimentary microfacies of the reservoir, the Es33 sub-member reservoirs for example were divided into first and second, and third classes corresponding to favorable exploration area and non-favorable exploration area (Fig. 13b).
Fig. 13.
Fig. 13.
Favorable exploration area of the Shahejie Formation sandstone in the Nanpu sag.
This study discusses the formation mechanisms of two sets of high-quality sandstone reservoirs in the same sag. Our study results show that, even though in the same sag, the formation mechanisms of different structural belts and layers are different to some extent, the favorable reservoirs should be predicted after we have a clear understanding of the formation mechanisms of the high-quality reservoir.
In conclusion, many factors affect the development of high-quality reservoirs in deep formations. The authors believe that the deep sandstone formations in petroliferous basins in eastern China still have great exploration potential. In the future, the spatial distribution of relatively high-quality reservoirs should be predicted according to the formation mechanisms of these reservoirs, to guide the efficient exploration of oil and gas.
4. Conclusions
The reservoirs of Es Formation in the Nanpu sag are mainly clastic sandstone. The Es1 sandstone in Nanpu No.3 structural belt depositing in braided delta front is dominated by lithic arkose, fairly rich in primary pore, and is low porosity but medium to high permeability reservoir. The Es3 in Gaoshangpu of fan delta front facies is dominated by feldspathic litharenite, rich in secondary dissolution pores locally, and belongs to tight sandstone reservoir, but there are favorable exploration areas.
The Es reservoir of the Nanpu sag is in the mesodiagenesis A stage, of which the Es1 reservoir in Nanpu3 structural belt is in the mesodiagenesis A2 stage, while the Es3 reservoir in Gaoshangpu structural belt is in the mesodiagenesis A1 stage. The reservoirs have experienced three types of diagenetic processes, compaction, cementation, and dissolution. Among them, compaction is the main factor causing reservoir porosity reduction, and cementation has certain destructive effect to the reservoirs, but dissolution greatly improves porosity of the reservoirs.
The formation mechanisms of deeply buried high-quality Es1 sandstone can be summarized as follow: pore preservation due to high compaction resistance ability is the dominant factor, while dissolution is the secondary factor. In contrast, dissolution is the major mechanism, sedimentary environment and compaction resistance ability of rigid particle are the secondary mechanisms for forming high-quality Es3 tight sandstone reservoir. Compaction resistance and dissolution are two major factors contributing to the development of high-quality clastic reservoirs in deep formations of strudy area. The oil and gas exploration in medium-deep formations should focus on coarse sediments in strong hydrodynamic environment and clastic reservoirs in high hydrocarbon expulsion intensity areas.
Nomenclature
C—cementation rate, %;
D—dissolution rate, %;
F—feldspar content, %;
H—rigid rock fragment content, %;
M—grain size, mm;
P—compaction rate, %;
Q—quartz content, %;
R—correlation coefficient, dimensionless;
RLLD—deep lateral resistivity log, Ω·m;
S—soft rock fragment content, %;
SC—sorting coefficient, dimensionless;
SP—spontaneous potential, API;
ϕ—porosity, %.
Reference
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Large and extra-large oil/gas fields are mainly distributed in Tethys areas, passive margins, foreland thrust belts, and craton basins in the world. Unconventional oil/gas fields are mainly distributed in foreland slopes, basin (depression) centers, craton synclines, and tundra. Since the 21st century, the major exploration discoveries across the globe have been mainly concentrated in the deep water area of passive margins, carbonate rock, lithologic-stratigraphic zone, foreland thrust belt, mature exploration area, new basin and unconventional oil/gas reservoir (field). These major discoveries involve conventional and unconventional oil/gas resources. The conventional oil geology stresses the oil/gas migration and reservoir-forming rules in individual traps; the unconventional oil geology focuses on unconventional resources, reservoir, reservoir-formation and technologies. The geological features, classification program, research content, evaluation method and exploration phase of unconventional oil/gas reservoirs (fields) are different from those of conventional ones. Research should be strengthened on unconventional oil geology to develop unconventional oil geological theories.
摘要
全球常规类大型、特大型油气田主要分布在特提斯域、被动陆缘、前陆冲断带和克拉通等盆地中。非常规类油气田主要分布于前渊斜坡、盆地（坳陷）中心、克拉通向斜区和冻土带等。21世纪以来全球油气勘探重大发现主要集中在被动陆缘深水区、碳酸盐岩、岩性-地层、前陆冲断带、成熟探区、新地区新盆地及非常规油气藏（场）等7大领域。这些重大发现涉及油气勘探中的常规与非常规2类油气资源。常规石油地质强调在单一明确圈闭中的油气运聚和成藏规律；非常规石油地质重点研究非常规资源、非常规储集层、非常规成藏与非常规技术等。非常规油气藏（场）在地质特征、分类方案、研究内容、评价方法和勘探阶段等方面与常规油气藏有明显不同，需要加强非常规石油地质研究，发展非常规石油地质理论。
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Diagenesis of deep sandstone reservoirs and a quantitative model of porosity evolution: Taking the third member of Shahejie Formation in the Wendong Oilfield, Dongpu Sag, as an example
High-quality source rocks in Nanpu Sag
Forming condition and enrichment mechanism of the Nanpu oilfield in the Bohai Bay Basin, China
Oil and gas exploration has achived appreciable progress in Nanpu sag with the highest oil and gas accumulation degree of Bohai Bay Basin, China. The oil and gas reserves has been much higher than the oil and gas resources evaluated before. It was thought that the total organic content(TOC) of source rocks in Nanpu sag is low, with the number of 0.8%-1.2%, which resulted in the low hydrocarbon resources. This research shows that high quality source rocks which is comparable with that of Shahejie Formation in Dongying sag [1, 2] exist in Nanpu sag and the hydrocarbon accumulation conditions are excellent.Organic matter-rich source rocks are identified within Shahejie Formation(Es), especially in Es34 member with the TOC up to 5%. It’s a suit of mature source rocks with the TOC generally more than 2%, S1+S2 more than 20mg/g, Tmax above 440℃. Laminas of organic matter-rich shale are developed, mainly including organic laminas and calcareous laminas, with kerogen and mineral interbeded. It’s easy to identify shale from surrounding rock according to obvious reflection enents and logging response of petrophysics.Laminated texture comporised of organic-rich clay layer and fine-calcareous laminas is the remarkable characteristics of Es34 formation. The horizontal clay layer, containing clay minerals and pyrite et al., is the deposition in standing water. The thickness of effective source rocks within Es34 formation is about 250m, and high quality source rocks around 100m. Ro is more than 0.8% and the organic matters are mainly from amorhpous mass and phycoplast. Source rocks within Es34 formation were formed in the early Eocene period when major faulted depression movement occurred. The lake basin occupied the largest area and water depth reached the highest level in history [3]; Widespread gray or black mudstone, calcareous mudstone and oil shales in thickness prevailed over the stage, which had great potential of hydrocarbon generation. Source rocks in Es33 formation is also of high quality, second only to that within Es34 formation. On the whole, source rocks in Es3 formation is the best, Es1 formation is next. At present, source rocks within Es34 formation have past its generation peak according to the burial depth of 5800m~6400m.Accumulation conditions in Nanpu sag is quite good. First, the Tertiary source rocks within Guantao formation had began to generate a large amount of hydrocarbon before the volcanic eruption which formed many hydrocarbon migration pathways like faults and contributed to hydrocarbon migration and filling. In addition, hydrocarbon enrichment factors are also based on kinds of migration pathways, multiple hydrocarbon source beds, multiple injection, compact balsalt cap rock and time-space matching(Fig. 1), especially the following two factors.(1) Igneous cap rock. Widespread basalt cap rock with large thickness is developed in Nanpu sag, with the thickness up to 500m, much of the region 50m, which other fault basins do not have.(2) Fault-based hydrocarbon migration pathways. Faults are the most important hydrocarbon migration pathways for every oil-bearing series in Nanpu sag, as well as the most important conditions for the multiple hydrocarbon accumulation. Many faults connected to source rocks or the basement rock are good migration pathways, so that sand bodies connected with the fault are full of hydrocarbons and there are many production layers in vertical.
Origin of the Paleogene deep burial high-quality reservoirs in the southern Nanpu Sag
Hydrocarbon distribution features and main controlling factors in the Nanpu Sag
hydrocarbon accumulation，oil and gas distribution in the Nanpu sag are mainly controlled by the following factors. Firstly，source rocks have controlled the distribution areas of hydrocarbon reservoirs. The study located in the place which is within 15 kilometers from shows more than 80% of hydrocarbon reservoirs have expulsion center.Secondly the favorable sedimentary fades have controlled the sand distribution and reservoir propertiesthe lays of hydrocarbon reservoirs and their scales. Finally，faults are the most important factor controlling hydrocarbon reservoirs' distribution. On the one hand faults have controlled thetrap types and their spatial distribution，further, the size and location of hydrocarbon reservoirs. On the ether hand,as channels for hydrocarbon's vertical migration，faults control their vertical migration distance.To sum up,the hydrocarbon distribution of the Nanpu sag is under the controlling effects of the combination of source rocks,sedimentary facies and faults.The deltas near the source rocks , channel sands and the fault zones interconnecting vertically are favorable exploration areas.
Study on the hydrocarbon mobility under the geological conditions of low porosity and permeability and prediction on movable potential in the Nanpu Sag
