Introduction
Among carbonate oil and gas fields across the world, oil/gas fields in Cretaceous rank first in number, accounting for 29%[1,2,3,4]. Characterized by high depositional rate, rudist shoal is dubbed tropical carbonate factory[5,6]. Previous studies focused more on the paleontology and paleoecology of rudist shoal[7,8,9], but less on the sedimentary features and diagenesis. The H Oilfield in Middle East is a giant oilfield dominated by bioclastic limestone reservoirs. The Upper Cretaceous Mishrif Formation is the primary pay zone, where the rudist shoal is an important reservoir type in Middle East[10,11,12]. However, its sedimentary diagenesis and control on the reservoir properties are not very clear, restricting efficient production and development.
In this study, based on core, cast thin section and whole rock analysis, conventional physical properties and high pressure mercury intrusion experiments, the sedimentary sequence of individual and complex rudist shoals, and diagenesis of the Cretaceous Mishrif Formation in the H Oilfield have been studied to sort out the control of sedimentary diagenesis on reservoir development, in the hope to provide geologic basis for exploration and development of carbonate reservoirs in Middle East.
1. Geological conditions
Located in the southeast, about 400 km from Baghdad, Iraq, and in the southern Mesopotamia Basin (Fig. 1), structurally, the H Oilfield is a NW-SE wide and gentle anticline on the whole, formed with the Neogene Zagros Orogeny [13]. Since the Cambrian period, the H block had long been located on the northern edge of Gondwanaland, when platform deposits developed. To the Cretaceous period, the carbonate rock of shallow marine shelf deposited with the weakening of tectonic activity.
Fig. 1.
Fig. 1.
Location of the study area.
Fig. 2.
Fig. 2.
Composite columnar section of H Oilfield (rudist shoal developed well sections). GR—Natural gamma; d—pore diameter; ρ—density; Δt —acoustic time difference.
2. Sedimentary characteristics of rudist shoal
2.1. Petrologic characteristics
The rudists boomed and once replaced coral as the primary main reef-forming organisms during the Cretaceous period, but died out in the Late Cretaceous period[22,23,24]. Since the colonization stage, the size of individual rudist increased from bottom to top. With only an umbo adhering to the base, they had poor stability. The rudist reef experienced biological, wave and ocean current erosion since formed, so there is hardly any large rudist reefs growing continuously, rather a large amount of rudist detritus developed[25,26].
The highly specialized form and shell composition make it easier to tell rudist detritus from other bivalves. There are gray elliptical and rectangular rudist detritus 2-8 cm in diameter visible on cores (Fig. 3a, 3b), which have dark and light growth striation under the microscope (Fig. 3c), and porous structures in orderly arrangement in some (Fig. 3d). The rudist detritus are the main grains in the rudist shoal, followed by biodetritus including echinoderms, non-sessile bivalves, benthic foraminifers, and bryozoans, and some rounded arene and pelletoids.
Fig. 3.
Fig. 3.
Cores and microscopic characteristics of rudist detritus in Mishrif Formation, H Oilfield.
Based on comprehensive analysis of cores collected from 8 wells and 368 cast thin sections of the Mishrif Formation, the rudist-shoal mainly consists of arene-rudist grainstone, rudist grainstone, rudist-gravel limestone, and some pelletoids-rudist packstone and a small amount of carbonaceous mudstone. It has grain content between 85% and 95%, good sorting, and low micrite content. Compared with the MB2-1, the MC1-4 has less crushed rudist detritus, more quite complete rudist detritus with porous structure and bryozoans.
The whole rock analysis of 68 cores shows the rudist shoals have predominant calcite but low dolomite content (Table 1). The low content of quartz and clay minerals, but no gypsum, anhydrite and rock salt, suggest warm, humid climate during the sedimentary period. The MC1-4 has lower calcite content and slightly higher contents of other minerals than the MB2-1.
Table 1 Whole rock analysis of rudist shoal of Mishrif Formation in H Oilfield.
| Core Layer | Mineral composition/% | Number of samples | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Quartz | K-feldspar | Plagioclase | Calcite | Dolomite | Siderite | Pyrite | Gypsum | Anhydrite | Rock salt | Clay minerals | ||
| MB2-1 | 0.21 | 0 | 0 | 99.06 | 0.73 | 0 | 0 | 0 | 0 | 0 | 0 | 46 |
| MC1-4 | 0.81 | 0 | 0 | 95.79 | 1.80 | 0 | 0.16 | 0 | 0 | 0 | 1.44 | 22 |
2.2. The sedimentary cycle
In the rudist shoal of the MC1-4 layer, there are 4 sedimentary cycles with grain sizes increasing and coarsening, thickness decreasing upward and variable lithology (Fig. 4). Cycle 1 at the bottom is the thickest, 2.61 m (Fig. 4a), and consists of low-dip cross-bedding pelletoids-rudist packstone, parallel bedding rudist grainstone and rudist gravel limestone (Fig. 4b-4d). Cycle 2 and cycle 3 in the middle show the evolution from rudist grainstone to rudist gravel limestone (Fig. 4e-4h), 1.93 m thick and 1.81 m thick respectively. Cycle 4 at the top is the thinnest, only 1.08 m, and made up of rudist grainstone and gravel limestone (Fig. 4i); and on the top is black carbonaceous mudstone 0.07-m-thick with horizontal bedding, which is covered by fore-shoal slope fine deposits in the next fourth-order sequence.
Fig. 4.
Fig. 4.
Core and cast thin section characteristics of rudist shoal in MC1-4 layer, Well M316, H Oilfield.
The MB2-1 rudist shoal has 8 reverse rhythm cycles (Fig. 5), with the thickness from 4.15 m to 1.45 m from bottom to top (Fig. 5a). Cycle 1 at the bottom has 1 m thick, low-dip, cross-bedding pelletoids and rudist packstone (Fig. 5b), and the other cycles mainly consist of parallel bedding arene - rudist grainstone, rudist grainstone and rudist gravel limestone (Fig. 5c-5i). Parallel complete elliptic rudist fossils were observed on cores and bryozoa detritus were observed on cast thin sections (Fig. 5e, 5f). Massive syndepositional limestone breccia at the top of the MB2-1 (Fig. 5i) is usually composed of framework organisms or gravels different from the bedrock, reflecting strong hydrodynamic force. It is covered by fine deposits of restricted platform inter-shoal in the next third - order sequence.
Fig. 5.
Fig. 5.
Core and cast thin section characteristics of rudist shoal in MB2-1 layer, Well M316, H Oilfield, Iraq.
2.3. Sedimentary sequence of individual rudist shoals
The carbonate sedimentary response has a close relationship with the fluctuation of sea level, which controlled the development characteristics and superimposition of rudist shoals[27,28]. The relative sea level during the deposition of the Mishrif Formation was characterized by frequent variation[14], and multiple reverse cycles exist in shoals of MC1-4 and MB2-1, reflecting multi-stage relative sea-level changes. Each cycle includes a rapid rising, a stable and a slowly dropping stage of the sea level. In the changing process of the relative sea level, the yield of platform carbonate sediments was not constant, but followed a S-shaped trend[29,30]. During the early flooding period, which was a sedimentary lag period, biological populations recovered and flourished on the platform, while carbonate rock deposited slowly. This period is called the incipient stage. When the carbonate factory fully developed, the yield of carbonate deposits increased, the relative sea level tended stable, the accommodation space was gradually filled. This period is called the complement stage. Later, when the deposition rate of carbonate increased further and became greater than the growth rate of the accommodation space, the accommodation space was rapidly filled. That period is called the simultaneity stage. As the simultaneity stage continued, the shoal top would expose[29,30,31,32].
The rudist shoal of the Mishrif Formation in the H Oilfield developed on the platform edge with strong hydrodynamic force, and is mainly composed of high-energy deposits of grain shoal facies. The relative sea level fluctuation, which changed the water depth and water turbulence, controlled the growth of rudist reef and deposition process of rudist shoal. According to the relative sea level, the lithologic evolution and sedimentary structure of the rudist shoal, the single rudist shoal is divided into four lithologic sections: A, B, C and D, corresponding to the slow deposition period, medium deposition period, rapid deposition period and exposure period (Fig. 6).
Fig. 6.
Fig. 6.
Complete sedimentary sequence of a single rudist shoal in Mishrif Formation, H Oilfield.
Section A was formed in the slow deposition period, when the relative sea level rose quickly, and rudists expanded and flourished, corresponding to the incipient stage of carbonate deposition. The rudist reef and its associated rudist shoal were near the wave base with a medium hydrodynamic force, and reefs and shoals grew slowly and formed pelletoids rudist packstone with low-dip cross-bedding. Section B was formed in the medium rate deposition stage, when the relative sea level was relatively stable, the yield of carbonate deposits increased, corresponding to the complement stage of carbonate deposition. The rudist shoal was above the wave base with a relatively strong hydrodynamic force, and the rudist reefs and shoals grew gradually and formed arene-rudist grainstone and rudist grainstone with low-angle cross-bedding and parallel bedding. Section C was formed in the rapid deposition period, when the relative sea level fell slowly, the accommodation space was rapidly filled, corresponding to the simultaneity stage of carbonate deposition. The rudist shoal was near the sea level, where strong hydrodynamic force and remarkable wave action brought abundant nutrients for rudist reefs and other suspension feeders, making them thrive and grow. Swashed by waves, reefs broke increasingly rapidly, forming massive synsedimentary breccia; meanwhile shoals grew rapidly, forming rudist gravel limestone with parallel bedding which is shown by strip rudist detritus arranged directionally in addition to grain size variation. Section D was formed in the exposure period, when rudist shoal emerged from seawater, the sedimentary environment turned into localized swamp with weak hydrodynamic force, consequently, the rudist reefs were restricted in growth and even died. Meanwhile plants began to grow and formed carbonaceous mudstone after buried.
In a complete rudist shoal, the lithology turns from packstone, grainstone, gravel limestone to carbonaceous mudstone. With the dropping of relative sea level from bottom to top, the rudist detritus increase, the grain size gets bigger, and grains in the upper part show parallel arrangement. The thickness of a single rudist shoal can indicate the sea level rise in a single period[21, 23]. When the relative sea level rise was small, rudist shoal would be long above wave base, as a result, the development degree of pelletoids-rudist packstone in Section A would be low. When the next sea level rise happened in the rapid deposition period, the shoal top was not exposed, carbonaceous mudstone of Section D would not develop. Moreover, during the generation of a later single shoal, the early one might be eroded, especially the top carbonaceous mudstone. Due to the above factors, in the development process of a shoal, the complete sedimentary sequence with sections A-D assemblage is rarely formed.
2.4. Assemblage of multiple shoals
Based on drilling and coring data, the rudist shoals of the MC1-4 and MB2-1 layers are composed of multiple rudist shoals superimposed vertically. They are in the high-stand systems tract and formed when the relative sea level dropped. As the rise rate of the sea level declined, the accommodation space decreased, the hydrodynamic force increased, and the thickness and lithological assemblage of the single shoal changed regularly (Fig. 7a, 7b).
Fig. 7.
Fig. 7.
Characteristics of lithologic assemblages of rudist shoals in Mishrif Formation of Well M316, H Oilfield.
In the initial depositional period of single shoal at the bottom of MC1-4 and MB2-1, the water was deep, the rudist shoal was near wave base with weak hydrodynamic and wave actions, there were some micrites in the deposits, forming pelletoids-rudist packstone. With the hydrodynamic force and wave action getting stronger, section B developed arene-rudist grainstone and rudist grainstone, and section C developed rudist gravel limestone with parallel bedding. With exposure of single shoals, section D developed carbonaceous mudstone. The single shoals at the bottom are thicker, and as the carbonaceous mudstone was seldom preserved due to flush later, shoals with the assemblage of A-B-C sections were generally formed.
During the deposition of single shoals in the middle, the rudist shoal was above the wave base where hydrodynamic force and wave action were strong, so the micrite content in deposits was relatively low. The pelletoids-rudist packstone of section A is often absent in a single shoal. The deposition of a single shoal generally started from section B, followed by section C and section D. The single shoal has medium thickness, poorly preserved carbonaceous mudstone of section D, and is often composed of B and C sections.
In the deposition of single shoals at the top, the water was shallow, the rudist shoal was above the wave base and close to the sea level, where strong hydrodynamic force and wave action made the deposits low in micrite content, and synsedimentary breccia size was large. With poor development of pelletoids-rudist packstone of section A, the deposition of a single shoal also often started from section B, followed by section C and section D. The single shoals at the top have small thickness, where the preservation of section D carbonaceous mudstone depends on the exposure duration and erosion intensity after deposition. On the top of the rudist shoal of the MC1-4 layer is a fourth-order sequence boundary. With a short exposure time, overlying slope micrite limestone, and late weak erosion, the carbonaceous mudstone of section D could be preserved partially. As a result, the single rudist shoal is often composed of B, C and D sections (Fig. 7a). At the top of the rudist shoal of MB2-1 is a third-order sequence, that means it had been exposed longer. Its overlying layer is inter-shoal wackestone, meaning medium strength erosion, so the carbonaceous mudstone of section D is hard to remain, and the single shoal is often made up of sections B and C (Fig. 7b).
3. Diagenetic evolution of rudist shoals
3.1. Diagenetic types and features
The observation and analysis results of cast thin sections show that the rudist shoals of the Mishrif Formation have experienced multiple kinds of diageneses from deposition to burial. The porosity enlargement diageneses include biological burrowing and atmospheric freshwater dissolution. The pore destruction diageneses include micritization, cementation and compaction. Among them, atmospheric freshwater dissolution, cementation and compaction have stronger influence on rudist shoal reservoirs.
The rudist shoal reservoir of the Mishrif Formation suffered severe atmospheric freshwater dissolution, which is the major pore enlargement diagenesis, including selective dissolution and unselective dissolution. The selective dissolution is mainly controlled by solubility difference [27]. In the initial stage of atmospheric freshwater leaching, unstable minerals were preferentially dissolved, forming moldic pores (Fig. 8a). Unselective dissolution happens after mineral stabilization and isn’t selective, which enlarges original pores and produces intergranular dissolved pores and dissolved caves (Fig. 8a-8c).
Fig. 8.
Fig. 8.
Diageneses of rudist shoals in Mishrif Formation of H Oilfield.
Microscopic analysis of cast thin sections shows that the cementation of the rudist shoal reservoirs in the Mishrif Formation is weak, and the cements were formed in seawater, atmospheric freshwater or burial environments. In a seawater environment, fibrous and foliated cements were formed on the surface of shells and grains due to the large seawater flow on the platform edge (Fig. 8d). In an atmospheric freshwater environment, cementation generally occurred in the freshwater undercurrent belt, where the calcium carbonate produced in the vadose zone precipitated, forming a small amount of equiaxed granular and coaxial accreted cements (Fig. 8e, 8f). In buried environment, cementation generated a small amount of iron-containing, partially purple coarse-grain calcite (Fig. 8g).
The rudist shoal reservoir of the Mishrif Formation had poorer resistance to compaction. Under the compaction of overlying strata, grains were usually in line contact or point-line contact, and were dislocated or separated. The long rudist shells broke into fragments that arranged directionally (Fig. 8h).
3.2. Diagenetic sequence and evolution
Based on the study of diagenetic types and characteristics, the diagenetic sequence and evolution of the rudist shoal reservoir in the Mishrif Formation in the H Oilfield have been established, and the stage of different diagenetic events and their effects on reservoir physical properties have been determined according to the relationship between mineral morphology, dissolution and cements, and combining with regional burial history (Fig. 9). The rudist shoal experienced syngenetic diagenesis, early diagenesis and intermediate diagenesis after deposition. In the early sedimentary stage, the rudist shoal was characterized by low content of micrite, coarse grains and rich primary pores (Fig. 10a), and diagenesis worked dramatically in the syngenetic diagenetic period. According to the diagenetic environment and geologic age, the syngenetic period is further divided into early, middle and late stages (Fig. 9).
Fig. 9.
Fig. 9.
Diagenetic sequence of rudist shoals in Mishrif Formation of H Oilfield.
Fig. 10.
Fig. 10.
Diagenetic evolution of rudist shoal of Mishrif Formation in H Oilfield.
In the early syngenetic diagenesis, microbes perforated rudist carbonate shells inward repeatedly, and the resulted pores were filled with micrites later, making porosity drop slightly (Figs. 8h and 10b). In addition, microbes bored rudist shells, forming a few isolated intragranular pores (Figs. 8g and 10b). In the rudist shoal with strong hydrodynamic force, waves drove supersaturated seawater to flow through the pore system rapidly due to the large seawater flow. With continuous contact between sediments and seawater, fibrous and foliated cements and other isopachous cements were formed on the surface of the shells and grains (Figs. 8d and 10b), which blocked pores and reduced porosity (Fig. 9).
In the middle syngenetic diagenesis, rudist shoals grew fast. During the deposition of the rudist gravel limestone in section C, as the deposition rate exceeded the growth rate of the accommodation space, sediments were likely to expose in atmospheric freshwater environment and suffer from short-term periodic leaching. The selective dissolution of non-sessile aragonitic bivalves generated moldic pores, when unstable cements formed in seawater environment basically disappeared, especially the aragonitic fibrous cements were completely dissolved. Most foliated cements were dissolved too (Fig. 10c), leading to a great rise of porosity. The dissolution in this stage influenced a relatively limited depth range, primarily the upper part of individual rudist shoals. The dissolution products provided solutes for re-precipitation in the freshwater undercurrent belt. A small number of pores were filled with equiaxed granular calcite (Fig. 8e), echinoderms and arenes were generally composed of calcite monocrystals, and cements grew coaxially around their outer cycle (Figs. 8f and 10c), reducing the reservoir porosity.
In the late syngenetic diagenesis, since the descending cycle of the sea level ended, the rudist shoal had experienced exposure on the whole for a long time after deposition. The foliated cements continued to dissolve with only a small amount remaining in the lower part of the rudist shoal. After unstable minerals were dissolved, atmospheric freshwater dissolution is mostly unselective. The relatively stable rudist and echinoderm fragments dissolved at this stage, resulting in uneven boundary of grains, embayed dissolution, enlargement of original pores, formation of intergranular dissolved pores, and increase of porosity in quite large magnitude. With further dissolution, the intergranular pores were enlarged into dissolved vugs greater than 1 mm in diameter (Figs 8a-8c and 10d), making the porosity rise further. In this stage, the dissolved depth was controlled by exposure period. Long exposure facilitated full and deeper freshwater leaching. The calcium carbonate produced from dissolution precipitated elsewhere, generating some equiaxed granular and coaxial accreted cements.
In the early diagenetic period, under the influence of “strong dissolution and weak cementation” in a shallow environment, the cement in the rudist shoal reservoir was little and unable to resist compaction. Moreover, some rudist detritus appeared as long strips and had irregular boundaries caused by unselective dissolution, which further led to weak anti-compaction ability of the rudist shoal. With overburden load, the pore fluid decreased, the sediment density increased, the grains were in line or point-line contact, and the porosity declined. Compaction made pelletoids deform into elliptical shape, while the long rudist detritus broke in the thinner part and exhibited directional arrangement (Figs. 8h and 10e). Although some intergranular microfractures were generated due to break of grains, the porosity increase was very limited, while compaction reduced the porosity to some extent.
In the middle diagenetic period, compaction went on under the stress of overly formation, but in smaller intensity than in the shallow buried environment. Local stress concentrating at the contact between grains resulted in pressolution, and accordingly concave-convex or sutured contact of grains (Figs. 8g and 10f), and porosity reduction. Coarse cements developed in some pores (Figs. 8g and 10f). In this period, the diagenesis wasn’t strong and was predominately destructive, leading to limited drop of porosity.
Due to the difference in morphology and composition of various grains in the rudist shoal (Fig 10a), the diagenetic characteristics of rudist shoal in marine, atmospheric freshwater and buried environments would be different (Fig. 10b-10f). The rudist detritus mainly underwent biological boring, atmospheric freshwater dissolution, equiaxed granular cementation, compaction and grain breaking. After the soft organic bodies inside the chamber decayed, benthic foraminifers mainly experienced micriticalization and equiaxed granular cementation. During the diagenetic process, bivalve detritus mainly went through selective dissolution and equiaxed granular cementation, echinoderm detritus and arenes mainly experienced unselective dissolution, coaxial accreted cementation and pressolution, and pelletoids suffered from compaction significantly, while bryozoa detritus mainly underwent equiaxed granular cementation. In general, the rudist shoal reservoir was characterized by “strong dissolution, weak cementation and strong compaction”, and deposition and dissolution made the greatest contribution to porosity, with primary pores and dissolved pores from meteoric fresh water making up the absolution majority of the pores. Meanwhile, the pores changes from single type to multiple types (Figs. 9, 10a and 10f).
4. Control of sedimentation and diagenesis on physical properties of rudist shoal reservoirs
4.1. Reservoir properties
The analysis of 368 cast thin sections, high-pressure mercury injection test results of 84 samples and 408 porosity and permeability test results of 408 samples shows that the rudist shoal of the Mishrif Formation in the H Oilfield contain largely intergranular pores, intergranular dissolved pores and moldic pores, and some dissolved caves. The mercury intrusion curve is obviously oblique, with displacement pressure mainly from 0.03 to 0.09 MPa. The pore throats are between 0.1 and 100.0 μm, and mostly coarse ones larger than 5 μm, characterized by an extremely wide peak (Fig. 11). The porosity mainly ranges between 20% and 24%, with an average of 21.9%, and the permeability is mainly (110-270)×10-3 µm2, with an average of 195.1×10-3 µm2 (Fig. 12). All these factors indicate that the rudist shoal reservoir of the Mishrif Formation has large pores, coarse throats and high permeability, so it is the major production layer in the H Oilfield.
Fig. 11.
Fig. 11.
Comparison of mercury injection curves and pore throat distribution curves of rudist shoal layers.
Fig. 12.
Fig. 12.
Comparison of porosity and permeability between rudist shoals and two layers (The single shoal is numbered the same as
One rudist shoal and different rudist shoals in one layer and rudist shoals in different layers are different in pore type, structure and permeability to some extent. Within one shoal, the upper part has more intergranular dissolved pores and dissolved caves (Fig. 4d), larger pore throats and better physical properties, while the lower part has higher proportion of residual intergranular pores (Fig. 4b), and poorer physical properties. For different rudist shoals, the shoal formed later has more dissolved pores (Fig. 4h), and larger slope of mercury intrusion curves, lower displacement pressure and median pressure, and higher proportion of coarse pore throats, simply put, better porosity and permeability (Fig. 12).
In the rudist shoals of the Mishrif Formation, the rudist shoals in MC1-4 layer differ more widely in physical properties, and the porosity and permeability show obvious reverse rhythm from bottom to top (Figs. 12 and 13). The rudist shoal in the MB2-1 layer has better developed dissolved caves and intergranular pores (Fig. 5e-5h), lower displacement pressure and median pressure, larger pore throat distribution range, more pore throats larger than 20 μm in diameter (Fig. 11), and less obvious reverse rhythm of porosity and permeability (Figs. 12 and 13).
4.2. Control on the reservoir
Comprehensive study shows that during the depositional stage of Mishrif Formation in the H Oilfield, the structure was relatively stable, and few fractures developed. The differences in the relative sea level, hydrodynamic force and diagenetic intensity led to different reservoir properties within the same shoal and between shoals, and between shoals in different layers (Fig. 13). Shoal 1 at the bottom is the thickest and formed in the most variable hydrodynamic condition. From bottom to top, the lithology changes from packstone of section A, grainstone of section B, to gravel limestone of section C. Located at the bottom, Shoal 1 was influenced little by late diagenesis.
Fig. 13.
Fig. 13.
Sedimentary diagenesis and reservoir physical properties of rudist shoals in MC1-4 and MB2-1.
During the deposition of a single shoal, the accommodation space was filled, the water depth decreased, and water energy increased gradually. There are some micrites in the lower part of single shoals. In the seawater environment, the supersaturated seawater flux was small, and the seawater cementation was relatively weak. In the atmospheric freshwater environment, neomorphism and dissolution were relatively weak, and cementation was relatively strong. In the buried environment, compaction and grain breaking were weakened due to support of cement (Fig. 4c). The upper part of single shoal has lower content of micrite and more primary pores, which is conducive to diagenetic reformation. In the seawater environment, seawater cementation was strong. In the atmospheric freshwater environment, more likely to expose, this part would be subject to stronger leaching, neogenesis and dissolution, and weak cementation during the penecontemporaneous period. In the buried environment, with poor ability to resist compaction, this part suffered severe compaction and fracturing (Fig. 4d), so the physical properties in a single shoal show reverse rhythm feature (Fig. 13).
Between different shoals, as the rising magnitude of relative sea level caused by high frequency cycles during the sedimentary period decreased, the depositional space of rudist shoals shrunk gradually, the hydrodynamic force during the deposition of later shoals increased, and the wave action turned stronger gradually. Therefore, from bottom to top, the shoals become thinner, micrite content decreases, the grain size turns bigger, and the synsedimentary breccia increases. The shoal formed later had more primary pores, and stronger cementation in the seawater environment. In the atmospheric freshwater environment, the shoal formed later, closer to the sea level, was more likely to suffer from atmospheric freshwater leaching and thus stronger neomorphism and dissolution, but weaker cementation. In the buried environment, it would subject to stronger compaction and grain breaking (Fig. 5h). In general, the shoals formed later have better physical properties (Figs. 12 and 13).
Beneath the same sequence boundary, the rudist shoal with more primary pores would have more developed dissolved pores and caves from unselective dissolution under simultaneous atmospheric freshwater leaching. Previously, the development depth of dissolved caves beneath the sequence boundary was defined as the range remarkably affected by atmospheric freshwater leaching. The range of a fourth-order sequence boundary is smaller, and that of a third-order sequence boundary is larger.
Among the rudist shoals of Mishrif Formation, the MC1-4 layer, depositing in the middle of the high-stand systems tract with a relatively weak hydrodynamic force, has higher micrite content. On the top of the rudist shoal is a fourth-order sequence boundary. Therefore, the exposure time of this layer was shorter, the depth of dissolved caves is only 3 m (Fig. 13a), the atmospheric freshwater leaching depth and reservoir reformation are limited, thus the differences in physical property inside and between single shoals are not smeared (Fig. 12). MC1-4 has slightly high porosity, because it contains a larger amount of rudist detritus with porous framework and bryophyte detritus with visceral foramen (Fig. 4f and 4g). The rudist shoal of MB2-1 is the best reservoir of the Mishrif Formation in the H Oilfield. During deposition, MB2-1 was at the top of the high-stand systems tract with a stronger hydrodynamic force, so it has lower micrite content. The strong wave action was more suitable for growth of rudist reefs and their corresponding shoals, so this layer of shoal is thicker. At the top of the rudist shoal is a third-order sequence boundary, that means it was exposed longer in atmospheric freshwater, so it suffered stronger unselective dissolution, with the dissolved pore-cave section amounting to 17 m deep (Fig. 13b). In other words, atmospheric freshwater leaching had wider influence. The whole shoal was subject to leaching, which weakened the difference in physical properties inside and between single shoals, so the reverse rhythm of porosity and permeability is less obvious (Fig. 12). This layer of rudist shoals has lower cement content, stronger compaction and grain breaking, more dissolved pores and intergranular pores, and displacement pressure only between 0.01 and 0.03 MPa.
Obviously, the deposition process determines the development location and basic characteristics of the rudist shoals of the Mishrif Formation. The sedimentary sequence of the rudist shoal is closely related to the growth and fragmentation of the rudist reef. The reef and shoal are both located in the high places of paleogeomorphology on the edge of platform with strong hydrodynamic force, and consist of largely grainstone and gravel limestone with low micrite content, rich intergranular pores, biological framework pores and other primary pores, so they have the material base to form excellent reservoirs.
Some diageneses further improved reservoir physical properties, in which exposure and atmospheric freshwater dissolution played key roles in reservoir development. Due to rapid accumulation, the shoals were subjected to atmospheric freshwater leaching frequently during the penecontemporaneous period. After depositing, the shoals underwent long- term atmospheric freshwater dissolution overall, so large amounts of intergranular dissolved pores, dissolved caves, intragranular pores, moldic pores and other secondary pores were generated, bringing about diverse reservoir spaces and pore structures.
5. Conclusions
There are rudist shoals in MC1-4 and MB2-1 of the Mishrif Formation, H Oilfield. The complete sedimentary sequence of a single rudist shoal usually includes a slow depositional period, an intermediate depositional period, a rapid depositional period and an exposure period, corresponding to A, B, C and D lithologic sections: pelletoids-rudist packstone, arene-rudist grainstone, rudist gravel limestone, and carbonaceous mudstone, respectively. In a multiple shoal complex, the single shoals from bottom to top decrease in thickness. Greatly influenced by dissolution of meteoric fresh water, cementation and compaction, the rudist shoal reservoir shows the characteristics of “strong dissolution, weak cementation and strong compaction”.
The depositional process determined the development position and material base of the rudist shoal, and the diagenesis further improved the physical properties. The rudist shoal reservoir mainly contains intergranular pores, intergranular pores, moldic pores, and some dissolved caves, has pore throats between 0.1 and 100.0 μm, mostly coarse pore throats larger than 5 μm. Therefore, the reservoir has large pores, coarse throats and high permeability. There are reverse lithological rhythm inside and between single shoals, with decreasing micrite content, strengthening dissolution, weakening cementation, increasing of pore throat size and physical properties getting better from bottom to top. The thickest rudist shoal of MB2-1 at the top of the high-stand systems tract subjected to the strongest atmospheric freshwater leaching and the most significant dissolution has the largest pores and throats, and is the best reservoir of the Mishrif Formation.
Reference
Introduction: Carbonate petroleum reservoirs
Characteristics of carbonate reservoirs for oil and gas fields in the world and essential controlling factors for their formation
Distribution patterns and controlling factors of giant carbonate rock oil and gas fields worldwide
Giant carbonate rock oil and gas fields are those whose reservoirs are made mainly of carbonate rock. Based on the statistical figures,at the end of 2012,a total of 1021 giant oil and gas fields were found worldwide,of which 321 ones were giant carbonate rock oil and gas fields. By means of statistical analysis,assessment of resources and sedimentology,this paper theoretically analyzes 226 giant carbonate rock oil and gas fields,focusing on their geographical locations,strata distribution,scale and buried depth of reservoirs,and types of traps. The analysis indicates that carbonate rock reservoirs are distributed mainly in the Persian Gulf Basin,the Gulf of Mexico Basin,Sirte Basin,Pre-Caspian Basin,North Slope of Alaska in the United States,Permian Basin,Sichuan Basin and Tarim Basin. Of those areas,the oil and gas resources of carbonate rock reservoirs are accumulated mainly in the Upper Paleozoic,Jurassic System,Cretaceous System,Paleogene System and Neogene System. Giant carbonate rock oil and gas fields can be classified mainly as biological reef,grain beach,dolomite,unconformity and weathering crust. Usually they are large in scale,with a buried depth of less than 3000 meters. However,the reservoirs with a large buried depth are mostly dolomite and high-pressure limestone. Based on the research on distribution patterns and accumulations of giant carbonate rock oil and gas fields,it is found that the current geophysical locations and vertical distribution of giant carbonate rock oil and gas fields are controlled by the carbonate rock plane and distribution of strata. The palaeoclimate and palaeolatitude controlled formation of hydrocarbon source rock and development of carbonate rock. The palaeostructure and its development controlled the scale of carbonate rock reservoirs and oil and gas abundance. The sedimentary diagenesis controlled reservoir functions of giant carbonate rock oil and gas fields while favorable source-reservoir-cap assemblage holds the key to formation of giant carbonate rock oil and gas fields.
New knowledge of hydrocarbon generating theory of organic matter in Chinese marine carbonates
DOI:10.1016/S1876-3804(17)30020-4
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The marine carbonates of the Tarim Basin are taken to study the relationship between source rock and oil and gas reservoirs and discuss the possibility and mechanisms of mature and low-TOC (less than 0.5%) marine carbonates being source rock. By studying the matching relationship between source rocks in Tarim Basin and hydrocarbon reservoirs in Tahe oilfield and analyzing the hydrocarbon-generating organism combination and its correlation with geochemical characteristics of oil and gas reservoirs, this study established a method to evaluate hydrocarbon generation substance of marine carbonate source rocks. It is concluded that the crude oil of the Tahe oilfield in the Tarim Basin is not derived from muddy source rocks, but has the obvious characteristics derived from carbonate source rocks. It revealed that the underestimated hydrocarbon-generating substance (organic acid salts) in the highly evolved marine carbonate rocks and high quality hydrocarbon-generating organism are the key to high-evolution carbonate rocks being as source rock. Organic acid salts have high hydrocarbon conversion rate and are mainly cracked into natural gas at high temperature. The development model of the source rocks of the Cambrian-Lower Ordovician carbonate source rocks in the Tarim Basin is dominated by the shelf model.
The morphological and ecological evolution of middle and upper Cretaceous reef- building rudistids
DOI:10.2307/3514530
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During the middle and Late Cretaceous, tectonoeustatic sealevel rise reached its highest Mesozoic-Cenozoic levels as a result of accelerated plate spreading and construction of positive topographic features on the seafloor (Haq et al, 1987). This resulted in the
The evolution of modern corals and their early history
DOI:10.1016/S0012-8252(02)00104-6
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Scleractinians are a group of calcified anthozoan corals, many of which populate shallow-water tropical to subtropical reefs. Most of these corals calcify rapidly and their success on reefs is related to a symbiotic association with zooxanthellae. These one-celled algal symbionts live in the endodermal tissues of their coral host and are thought responsible for promoting rapid calcification. The evolutionary significance of this symbiosis and the implications it holds for explaining the success of corals is of paramount importance. Scleractinia stands out as one of the few orders of calcified metazoans that arose in Triassic time, long after a greater proliferation of calcified metazoan orders in the Paleozoic. The origin of this coral group, so important in reefs of today, has remained an unsolved problem in paleontology. The idea that Scleractinia evolved from older Paleozoic rugose corals that somehow survived the Permian mass extinction persists among some schools of thought. Paleozoic scleractiniamorphs also have been presented as possible ancestors. The paleontological record shows the first appearance of fossils currently classified within the order Scleractinia to be in the Middle Triassic. These earliest Scleractinia provide a picture of unexpectedly robust taxonomic diversity and high colony integration. Results from molecular biology support a polyphyletic evolution for living Scleractinia and the molecular clock, calibrated against the fossil record, suggests that two major groups of ancestors could extend back to late Paleozoic time. The idea that Scleractinia were derived from soft-bodied, “anemone-like” ancestors that survived the Permian mass extinction, has become a widely considered hypothesis. The 14-million year Mesozoic coral gap stands as a fundamental obstacle to verification of many of these ideas. However, this obstacle is not a barrier for derivation of scleractinians from anemone-like, soft-bodied ancestors. The hypothesis of the ephemeral, “naked coral”, presents the greatest potential for solution of the enigma of the origin of scleractinians. It states that different groups of soft-bodied, unrelated “anemone-like” anthozoans gave rise to various calcified scleractinian-like corals through aragonitic biomineralization. Although there is evidence for this phenomenon being more universal in the mid-Triassic interval, following a lengthy Early Triassic post-extinction perturbation, it appears to have occurred at least three other times prior to this interval. This idea suggests that, because of ephemeral characteristics, the skeleton does not represent a clade of zoantharian evolution but instead represents a grade of organization. In the fossil record, skeletons may have appeared and disappeared at different times as some clades reverted to soft-bodied existence and these phenomena could account for notable gaps in the taxonomic and fossil record. A fuller understanding and possible solution to the problem of the origin of modern corals may be forthcoming. However, it will require synthesis of diverse kinds of data and an integration of findings from paleobiology, stratigraphy, molecular biology, carbonate geochemistry, biochemistry and invertebrate physiology.
Cretaceous world and oceanic red beds
The Cretaceous represents one of the extreme greenhouse climate in the geologic history of our planet . As such the Cretaceous is considered as a representative study topic in Earth system sciences. In this paper we firstly review the latest knowledge and new developments in the studies of the Cretaceous sedimentation, palaeoclimatology, and palaeoceanography, and attempt to provide a more realistic picture of the greenhouse Cretaceous world. Secondly, we summarize the Cretaceous anoxic events and the associated deposition of black shale. This is arguably one of the hottest topics in Cretaceous research over the past 30 years. The Cretaceous Oceanic Red Beds (CORB) and its oxic processes, brought forward for the first time by Chinese scholars. The Cretaceous anoxic events and CORB are closely related to the carbon and oxygen cycles. These will serve in future as key topics for better understanding the Cretaceous world, especially the palaeoclimatology and palaeoceanography.
Petroleum system analysis of the Mishrif reservoir in the Ratawi, Zubair, North and South Rumaila oil fields, southern Iraq
The rise and fall of Rudist reefs
DOI:10.1511/2002.2.148
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Abstract Available from
The nature and origin of Upper Cretaceous basin-margin rudist buildups of the Mesopotamian Basin, southern Iraq, with consideration of possible hydrocarbon stratigraphic entrapment
DOI:10.1016/j.cretres.2004.11.016
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The Cenomanian Mishrif Formation is the main carbonate Cretaceous reservoir in southern Iraq. The reservoir units of the formation consist of bioclastic and peloidal limestones, derived mainly from rudist banks within the formation. The banks are found to be of two types, either basin margin buildups occupying a narrow belt along the present day Iraq-Iran border, or as patchy buildups on the crestal parts of the giant structures such as at West Qurna, Rumaila and Zubair. These buildups were eroded as they shallowed to reach the wave-base zone. Data from the Dujaila Field in southern Iraq suggest that these buildups may act as stratigraphic traps that produced oil from a relatively structurally lower well, while the higher well was found to be dry. This finding is of significant exploration value and may prove the existence of large hydrocarbon accumulations in the Mishrif (as well as the Shu'aiba and Mauddud formations) in areas beyond the known structures.
Mid-Cretaceous rudists (Bivalvia: Hippuritida) from the Langshan Formation, Lhasa block, Tibet
DOI:10.1002/spp2.1019
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Abstract The Cretaceous Langshan Formation, which crops out along the northern portion of the Lhasa block, is composed mainly of grey or grey-black fine-grained limestones, argillaceous limestones and a few thin-bedded siltstones and silty mudstones. From specimens collected and/or photographed in the field, combined with a revision of taxa previously recorded from the Langshan Formation, four rudist taxa are recognized: (1) Auroradiolites gen. nov.; (2) Eoradiolites cf. davidsoni ; (3) Magallanesia rutogensis sp. nov.; and (4) Sellaea sp. We propose Auroradiolites gen. nov. for the grouping of SW Asian to Pacific radiolitid species formerly attributed to Eoradiolites and characterized by a compact (non-celluloprismatic) outer shell layer. The new genus is represented in the Langshan Formation by A. biconvexus (Yang et al ., 1982), including several other synonymized taxa. Taxonomic confusion in the literature has resulted from the varied appearance of the strongly developed ligamentary infolding in such forms: in the right valve, it has a forked inner termination, but common transverse sections through both valves show the latter juxtaposed against its broad, rounded counterpart in the left valve, giving the combined terminations a bulb-shaped appearance. Associated Eoradiolites cf. davidsoni is of smaller size and, by contrast, shows the celluloprismatic structure of the outer shell layer. The recently described canaliculate polyconitid genus, Magallanesia Sano et al ., 2014, is represented by M. rutogensis sp. nov., which differs from the type species in possessing more numerous and ventrally extended canals in the left valve. Possibly belonging with the latter species are isolated right valves, similar to Horiopleura , but containing tabulae. A probable Sellaea sp. is represented by poorly preserved right valves nevertheless showing typical features, including a very thin outer shell layer and an erect posterior myophoral wall, separating off a broad posterior ectomyophoral cavity. Based on the presence of Magallanesia and the orbitolinid record, this rudist fauna is probably no older than late Albian, while a younger age cannot be excluded. Both Magallanesia and Auroradiolites are characteristic of a SW Asian/central Pacific faunal province, while the other two genera are more widely distributed. .overlined { text-decoration: overline; } .struck { text-decoration:line-through; } .underlined { text-decoration:underline; } .doubleUnderlined { text-decoration:underline;border-bottom:1px solid #000; }
Geological features of grain bank reservoirs and the main controlling factors: A case study on Cretaceous Mishrif Formation, Halfaya Oilfield, Iraq
DOI:10.1016/S1876-3804(16)30047-7
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The geological features and distribution regularity of the grain shoal reservoirs of the Cretaceous Mishrif Formation in Halfaya Oilfield and their main controlling factors were analyzed based on cored data, 302 pieces of thin sections and 2507 experiment data points. Various types and multiple phases of grain shoal reservoirs are developed in Mishrif Formation, which are dominated by grainstone, followed by wackestone packstone. The formation and distribution of grain shoal reservoirs are mainly controlled by relative sea level change, paleogeomorpholoy and contemporaneous-penecontemporaneous dissolution. Relative sea level change controls the types and development phases of grain shoals. When the sea level falls, rudists, coquina, arene and pelletoids are extensively deposited, among which, rudists clastic shoal and coquina shoal that are deposited at the lowest sea level have the best reservoir properties, with a maximum porosity of 33.94%, maximum permeability of 764.571×10613μm2and an average permeability of 45.81×10613μm2. Paleogeomorpholoy controls high-energy facies belt and the dissolution of meteoric water. The submarine paleohigh controls the formation and distribution of high-energy facies belt during depositional period. However, during uplifting and denudation period, the reservoirs in paleohigh are intensively dissolved and represent good reservoir properties, with an average porosity of 14.78% and permeability of 7.849×10613μm2. Contemporaneous-penecontemporaneous dissolution results in the formation of most effective pores, which is the key factor for the formation of grain shoal reservoirs in the study area.
Shoal facies reservoir characteristics and genesis of Mishrif Formation in Halfaya Oilfield, Iraq
DOI:10.13278/j.cnki.jjuese.201704104
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The Mishrif Formation in the Halfaya oilfield of Iraq develops shoal facies reservoir,and it is one of the main producing horizons in the study area.The Mishrif Formation sedimentary facies has been studied using the data of core,thin section and well logging.The result shows that four types of sedimentary facies are developed in the Mishrif Formation,which are restricted platform,open platform,intra-platform and margin platform.The shoal facies reservoirs are developed mainly in the facies of open platform and margin platform.Casting thin sections observation,physical experiment,and mercury injection are conducted to study the characteristics and genesis of shoal facies reservoirs.A shoal facies reservoir has multiple types of rocks,including bioclast limestone,bioclastic packstone/wackestone and calcarenite etc.,and the rock particles are thicker.The pore types are various,mainly in the form of moldic pore,intergranular pore,dissolved pore and visceral foramen,of which intergranular pore and moldic pore development are the most widespread.To all kinds of shoal facies reservoirs,the porosities are higher,the permeabilities differ distinctly with the average permeability at(2.35~139.17) 10~(-3) m~2.The overall pore structure is relatively good,displacement pressure is low,but large throats are developed.The development of shoal facies reservoirs is controlled by the depositional environment and diagenetic transformation degree influence.A favorable sedimentary condition is the foundation of the shoal facies reservoirs development,and syngenetic period dissolution modification improves the reservoir properties,so that it become a high quality reservoir.
Characteristics of depositional environment and evolution of Upper Cretaceous Mishrif Formation, Halfaya Oilfield, Iraq based on sedimentary microfacies analysis
DOI:10.1016/j.jafrearsci.2018.01.007
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As one of the most important carbonate targets in the Middle East, Upper Cretaceous Mishrif Formation has been highlighted for a long time. Although consensus has been reached on the overall sedimentary background, disputes still exist in understanding the sedimentary environment changes among sub-regions due to relatively limited research, rare outcrop, and incomplete drilled core, which hinders the analysis on sedimentary environment and thus the horizontal and vertical correlation. In this study, taking the Halfaya Oil Field as an example, the sedimentary microfacies analysis method was introduced to comprehensively characterize the cored interval of Mishrif Formation, including Single Layers MC1-1 to MA2. A total of 11 sedimentary microfacies are identified through system identification of sedimentary microfacies and environmental analysis, with reference to the standard microfacies classification in the rimmed carbonate platform. Then three kinds of environments are identified through microfacies assemblage analysis, namely restricted platform, open platform, and platform margin. Systematic analyses indicate that the deposits are mainly developed in the open platform and platform margin. Meanwhile, rock-electricity interpretation model is established according to the electricity response to cored intervals, and is then employed to interpret the uncored intervals, which finally helps build the sedimentary evolution pattern through horizontal and vertical correlation. It is proposed that the Single Layers MC1-1 to MB2-3 were deposited in the open platform featured by low water level, including sub-environments of low-energy shoal within platform and inter-shoal sea; Single Layers MB2-2 to MB1-2B were deposited in the open platform and platform margin, including sub-environments of high-energy shoal on the platform margin, low-energy shoal within platform, inter-shoal sea, and open sea; and Single Layers MB1-2A to MA2 were again deposited in the open platform with high water level, and the circumstance of open sea was dominant. The deposition of Single Layers MC1-1 to MA2 actually corresponded to a retrogradation-progradation process. Results of this study will not only provide significant guidance to the exploration and development of Mishrif Formation, Halfaya Oil Field, but also support that the theory of sedimentary environment correlation with adjacent areas is reliable.
Identification of facies- controlled eogenetic karstification in the Upper Cretaceous of the Halfaya oilfield and its impact on reservoir capacity
Pore structure characteristics and control factors of carbonate reservoirs: The Cretaceous Mishrif Formation, Halfaya Oilfield, Iraq
Sedimentological characterization of the mid-Cretaceous Mishrif reservoir in southern Mesopotamian Basin, Iraq
Sequence stratigraphic analysis of the mid-Cretaceous Mishrif Formation, southern Mesopotamian Basin, Iraq
DOI:10.1111/jpg.12584
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The middle Cenomanian early Turonian Mishrif Formation, a major carbonate reservoir unit in southern Iraq, was studied using cuttings and core samples and wireline logs (gamma-ray, density and sonic) from 66 wells at 15 oilfields. Depositional facies ranging from deep marine to tidal flat were recorded. Microfacies interpretations together with wireline log interpretations show that the formation is composed of transgressive and regressive hemicycles. The regressive hemicycles are interpreted to indicate the progradation of rudist lithosomes (highstand systems tract deposits) towards distal basinal locations such as the Kumait, Luhais and Abu Amood oilfield areas. Transgressive hemicycles (transgressive systems tract deposits) represent flooding of the shallow carbonate platform and are recorded in oilfields such as Amara, Halfaya and Zubair.A sequence stratigraphic framework has been constructed for the Mishrif Formation based on correlation of the transgressive and regressive hemicycles which are separated by maximum flooding surfaces. Three third-order sequences are identified which show lateral and vertical facies variations depending on relative sea-level changes. Sequence boundaries are characterized by karstic, exposure and drowning features. Middle Cenomanian Turonian eustatic sea-level changes together with regional-scale tectonic deformation of the Arabian Plate controlled the availability of accommodation space and therefore the depositional profile during development of each sequence. Both of these factors controlled the maximum flooding surfaces and sequence boundaries which have been identified.The sequence stratigraphic key surfaces presented in this study represent typical candidate horizons or datum surfaces for future seismic or resevoir modelling studies. Also, lateral facies variations in each transgressive-regressive sequence and associated carbonate bodies (i.e. prograding shelf margin, forced regressive wedge) may form important stratigraphic traps in the Mesopotamian Basin.
Role of facies diversity and cyclicity on the reservoir quality of the mid-Cretaceous Mishrif Formation in the southern Mesopotamian Basin, Iraq
Facies development and paleoecology of rudists and corals: An example of Campanian transgressive sediments from northern Croatia, northeastern Slovenia, and northwestern Bosnia
DOI:10.1007/s10347-015-0451-7 URL [Cited within: 1]
Calcite/ aragonite ratio fluctuations in Aptian rudist bivalves: Correlation with changing temperatures
DOI:10.1130/G37389.1
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ABSTRACT Understanding how bivalves responded to past temperature fluctuations may help us to predict specifc responses of complex calcifers to future climate change. During the late-Early Aptian, aragonite-rich rudist bivalves decreased in abundance in northern Tethyan carbonate platforms, while rudists with a thickened calcitic outer shell layer came to dominate those of Iberia. Seawater cooling and variations in calcium carbonate saturation states may have controlled this faunal turnover. However, our understanding of how rudist lineages responded to changing environmental conditions is constrained by a lack of quantitative data on the evolution of thickness, size, and mineralogy of the shell. This study is based on volumetric measurements of the shell and shows the transition in lineages of the family Polyconitidae from aragonite-rich mineralogy in the earliest Aptian, to low-Mg calcite-dominated mineralogy in the middle Aptian, returning to aragonite-dominated composition in the latest Aptian. The platform biocalcifcation crisis that occurred at the Early-Late Aptian boundary in the Tethys was marked by a relative increase of calcite and a decrease in skeletal thickness and commissural diameters. The highest calcite/aragonite (Cc/A) ratios in polyconitid rudists accompanied the late Aptian cold episode, and the lowest values were reached during the warmer intervals of the earliest and latest Aptian. These results imply a correlation between Cc/A ratio values and temperature and suggest that some bivalves adapted to less favorable calcifcation conditions by changing calcite and aragonite proportions of their bimineralic shells and decreasing skeletal thickness, thereby reducing the metabolic cost of shell growth.
Carbonate facies in geologic history
DOI:10.1007/978-1-4612-6383-8
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Home. Slovenian English. ZRC SAZU; Presentation; Mission; Institutes; Information. Fran Ramov08Institute of the Slovenian Language Institute of Archaeology Milko Kos Historical Institute FranceStele Institute of Art History Institute of Musicology Institute of the Slovenian Literature
Reservoir and early diagenesis characteristics of rudist shoal of Mishrif Formation in HF Oilfield of Iraq
DOI:10.3969/j.issn.1673-5005.2016.05.001
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Based on observations of cores and thin sections,and measurements of trace elements and isotope,the microfacies and early diagenesis of MB2-MC1 member in Mishrif Formation,HF Oilfield of Iraq were studied. The sedimentary facies of MB2-MC1 member were open platform and shelf margin. There were two layers of abundant rudists deposited in rudists shoal,and two other layers deposited by storm. The rudist shoal was deposited in shelf margin,where the Sr/Ba vaule was 5. 74,higher than that of the rudist layers deposited by storm. The rudist shoal shows the characteristics of nonindigenous growth and strong synsedimentary breakup,which are very different from other reefs. The main lithofacies of rudist shoals were composed of rudist rudstone,rudist grainstone and rudist packstone,without reef limestone. The rudist shoals alternate with open shelf shoals. The meteoric water leaching of rudist shoals was strong during exposure,with weak cementation. The pore assembly of rudist shoals was composed of intergranular pores,intergranular dissolved pores,cave and dissolved microfractures.
Geologic features and genesis of the barriers and intercalations in carbonates: A case study of the Cretaceous Mishrif Formation, West Qurna oil field, Iraq
DOI:10.1016/S1876-3804(16)30018-0
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The type, poroperm relationship, log response characteristics and identification criteria of the barriers and intercalations in the carbonates of the Cretaceous Mishrif Formation in the West Qurna oil field of Iraq are studied through comprehensive analysis of cores, thin sections and well logs. The genesis and distribution of the barriers and intercalations are analyzed from the perspectives of sequence stratigraphy, depositional facies and diagenesis. The barriers and intercalations can be classified into three types: grainstone, packstone and wackestone. The barriers and intercalations generally exist in restricted platform facies and evaporative platform facies, some in open platform. They are common in transgressive cycles and early regressive cycles, forming continuously, extensive barriers and intercalations near the sequence boundaries. Penecontemporaneous cementation, burial compaction, and burial cementation led to the decrease of the porosity, damaged the space of the reservoir pores and became the important factors for the genesis of the barriers and intercalations. In the epidiagenetic phase, a large number of CaCO3precipitated in the phreatic water zone below free-water table, leading to the formation of packstone and grainstone barriers and intercalations in a large scale. Through comprehensive log analysis, log identification criteria and the distribution of the barriers and intercalations are determined. The barriers are mainly distributed in the sections of CRI and CRII, and the intercalations are concentrated in the section of mB1. For different types of barriers and intercalations, packstones are the most in quantity, followed by grainstones and wackestones.
Influences of eustatic movement on the Cambrian reservoirs of bank facies in Weiyuan gas field, the Sichuan Basin
Reef response to sea level rise: Keep-up, catch-up or give-up//Proceedings of the Fifth International Coral Reef Congress
Stratigraphic Response of a carbonate platform to relative sea level changes: Broken ridge, southeast Indian Ocean: Discussion (1)
Highstand shedding of carbonate platforms
Reef composition and their forming models of Changxing Formationin Jiantianba section of Lichuan, Western Hubei
DOI:10.1007/s11783-011-0280-z
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Jiantianba of Lichuan,western Hubei,is located in platform margin facies between carbonate platform facies in the eastern Sichuan and Western Hubei Trough.In the study area,the water gradually shallowed from the bottom to up in the stage of Changxing Formation of Late Permian,and developed a huge scale platform marginal reef deposition in aggradation-progradation.Based on the multidisciplinary analysis of measured profiles,the authors discussed the reef composition and forming model of Changxing Formation reef in Jiantianba section.The study demonstrated that the study area developed two third-order sequences in Changxing depositional stage of late Permian and the body of reef in the Jiantianba of Lichuan was located in HST of sequence 1of Changxing Formation.And its internal constituted by the four parasequence sets.Pss1 mainly composed by dark grey lamellar or lamellar-middle bedded cryptite,clastizoic cryptite,middle bedded bioclastic cryptite,the internal could be further divided into three parasequences namely PS2,PS3 and PS4.PS2 and PS3 were located in PSS1 lower-middle part which are composed by a set of slope facies of lamellar dark gray mudstone deposit,and PS4 was located at the top of PSS1 and developed a set of gray middle lamellar bioclastic limestone with the shoal facies deposition and the reef of Jiantianba was developed on the basis of the above bioclastic bank.PSS2 was a platform edge bioherm deposition,developed a set of grey massive algae bonding,algal bonding-baffle and baffle bioconstructed limestone,this parasequence set internal developed three parasequences,and PS5 mainly was algae bondstone,the lower part of PS6 and PS7 mainly were algae bondstone,the upper part of PS6 and PS7 was algae bond-bafflestone,which reflects the water shallow gradually,hydrodynamic stronger and bioherm prosperous gradually.PSS3 is mainly composed by a set of gray massive baffle-framework reef limestone,sponge organic framework reef and partly limestone breccia,the internal can be further divided into three parasequences namely PS8 PS9 and PS10;With relative sea level further drops,frame builders extremely flouring and mainly are Sphinctozoa,Inozoa and little Sclerospongiae,Hydrozoa and Bryozoan,this period was flourishing period of reefs,this parasequence set constitutes the main deposition of platform margin reefs.PSS4 constituted by a medium-bedded and light gray bioclastic limestone with dolomite and remains of bioclastic dolomite,and belonged to platform edge and restricted platform deposition.As the relative sea level fell gradually and hydrodynamic force strengthening progressively,the reef in the Jiantianba of Lichuan experienced four stages of evolution,including a foundation,the initial flourishing,prosperous,and the largest decline of organic reef.Reefs in Jiantianba of Lichuan reservoir system mainly distributed at the top of the HST,the environment for the formation include platform edge bioherm,platform edge reef and restricted platform dolomite deposition at the top of reef,high quality reservoir is closely related to the reef composition and deposition conditions.
