Facies Belts Research Articles

Types of the Cambrian platform margin mound-shoal complexes and their relationship with paleogeomorphology in Gucheng area, Tarim Basin, NW China

Based on the observation of outcrops and cores, thin section identification, restoration of paleogeomorphology by residual thickness method, fine description of seismic facies, etc., the coupling relationships between the development patterns of various types of Cambrian platform margin mound-shoal complexes and paleogeomorphology in the Gucheng area of Tarim Basin have been examined. The Cambrian platform margin mound-shoal complex is divided into mound base, mound core, mound front, mound back and mound flat microfacies, which are composed of dolomites of seven textures with facies indication. The different paleogeomorphology before the deposition of mound-shoal complex in each period was reconstructed, and three types of mound-shoal complex sedimentary models corresponding to the paleogeomorphologies of four stages were established: namely, the first stage of gentle slope symmetric accretion type, the second stage of steep slope asymmetric accretion type and the third and fourth stages of steep slope asymmetric progradation type. Their microfacies are respectively characterized by “mound base - mound back + (small) mound core + mound front - mound flat” symmetric vertical accretion structure, “mound base - (large) mound core + mound front - mound flat” asymmetric vertical accretion structure, “mound base - (small) mound core + mound front - mound flat” asymmetric lateral progradation structure. With most developed favorable reservoir facies belt, the steep slope asymmetric accretion type mound-shoal complex with the characteristics of “large mound and large shoal” is the exploration target for oil and gas reservoir.

Discovery of carbonate source rock gas reservoir and its petroleum geological implications: A case study of the gas reservoir in the first member of Middle Permian Maokou Formation in the Fuling area, Sichuan Basin

Carbonate rocks in the first member of the Middle Permian Maokou Formation in the Sichuan Basin (hereinafter “Mao 1 Member” for short) have been taken as a set of carbonate source rocks, and they have not been specifically studied from the aspects of reservoir evaluation and testing. By referring the exploration ideas of unconventional natural gas, the Mao 1 Member in Fuling area of southeastern Sichuan Basin has obtained industrial gas flow in development wells in recent years. In order to further clarify the natural gas exploration potential of the Mao 1 Member in this area, it is necessary to study its sedimentary characteristics, natural gas accumulation conditions and main control factors based on the data of field section measurement, systematic coring in drilling and laboratory testing. The following research results were obtained. First, the gas reservoir in the Mao 1 Member in the Fuling area is of source–reservoir integration, and its natural gas is mainly enriched in blackish gray marlite and nodular marlite. Second, its reservoir spaces are dominated by grain margin pores (fractures), diagenetic shrinkage pores (fractures), organic pores and fractures. Third, the pores are mostly in nanometer scale, and the main pore diameter is in the range of 5–50 nm, which is between shale reservoir and conventional reservoir, with strong heterogeneity. Fourth, the gas reservoir is characterized by source–reservoir coexistence, lithology controlling reservoir and extensive layered distribution, presenting two-stage differential hydrocarbon enrichment, namely intraformational near-source enrichment in the early stage and interformational blowdown adjustment in the late stage. Fifth, the development of blackish gray organic-rich fine marlite which is deposited with the episodic upwelling in the outer ramp facies belt is the foundation for the natural gas accumulation, the transformation of clay minerals controls the development of higher-quality reservoirs, good preservation conditions are the key to the natural gas accumulation, and fracture development is favorable for the enrichment and high yield of natural gas. In conclusion, the Mao 1 Member in this area is a special type of gas reservoir, i.e., carbonate source rock gas reservoir, which has greater potential of natural gas exploration and industrial gas flow have been obtained in several wells. The discovery of such type of gas reservoirs not only expands the domain of natural gas exploration in the Sichuan Basin, but provides the reference for the natural gas exploration in other areas of South China.

New progress and prospect of Middle Permian natural gas exploration in the Sichuan Basin

In recent years, natural gas exploration in the deep marine carbonates of the Sichuan Basin has been strengthened continuously. And based on the research results in the regional geological setting, a great number of new geological understandings are obtained in terms of lithofacies palaogeography, reservoir genesis and hydrocarbon accumulation evolution. What's more, new progress of natural gas exploration is achieved in many strata and domains in the Sichuan Basin, e.g. Qixia Fm of Middle Permian, Maokou Fm and Emeishan basalt, and the prospect of natural gas exploration is promising. In this paper, the recent exploration achievements of Middle Permian natural gas in the Sichuan Basin were summarized. First, under the control of palaeogeomorphology of late Caledonian, marginal platform shoal and girdle intra-platform shoal that are distributed in a large scale were developed at the edge of western Sichuan sea basin and in the periphery of central Sichuan paleo-uplift during the sedimentation of Qixia Fm, and they are large in scale and extensive in distribution range. Dolomite reservoirs are developed with good gas bearing property. Second, two kinds of reservoir bodies are mainly developed in the Maokou Fm of the Sichuan Basin. One is the shoal body which is developed along the marginal platform and the intra-platform high belt under the control of facies belt superimposed with late dolomitization, and the other is the effective karst reservoir which is distributed at the karst slope belt under the action of large-scale erosion in the late Maokou period. Third, volcanic gas reservoirs are discovered in the areas of Chengdu and Jianyang, and their quality reservoirs are of volcanic eruption facies and characterized by great thickness, good physical properties and extensive distribution. The discovery of this new type of gas reservoir expands the exploration domain of Middle Permian natural gas in the Sichuan Basin. In conclusion, the improvement of the large-scale natural gas exploration potential in many strata and domains of Middle Permian in the Sichuan Basin promotes it to be one important replacement domain of the further increase of natural gas production and reserves in the Sichuan Basin.

Geological conditions, reservoir evolution and favorable exploration directions of marine ultra-deep oil and gas in China

By analyzing the structural background, petroleum geological conditions, and typical regional (paleo) oil and gas reservoirs in marine ultra-deep oil and gas regions in China, this paper reveals the evolution processes of the marine ultra-deep oil and gas reservoirs and the key controlling factors of accumulation. The marine ultra-deep oil and gas resources in China are buried at depth of greater than 6000 m, and are mainly distributed in the Precambrian and Lower Paleozoic strata in the Sichuan, Tarim and Ordos cratonic basins. The development of marine ultra-deep source rocks in China is controlled by cratonic rifts and cratonic depressions with the background of global supercontinent breakup-convergence cycles. The source rocks in Sichuan Basin have the most developed strata, followed by Tarim Basin, and the development strata and scale of Ordos Basin needs to be further confirmed. The marine ultra-deep reservoir in China is dominated by carbonate rocks, and the reservoir performance is controlled by high-energy sedimentary environment in the early stage, superimposed corrosion and fracture in the later stage. The regional caprocks are dominated by gypsum salt rocks, shale, and tight carbonate rock. The ultra-deep oil and gas fields in China have generally experienced two stages of oil-reservoir forming, cracking (or partial cracking) of paleo-oil reservoirs, and late finalization of cracked gas (or highly mature to over mature oil and gas). The oil and gas accumulation is controlled by static and dynamic geological elements jointly. Major hydrocarbon generation center, high quality and large-scale reservoir resulted from karstification of high energy facies belt, thick gypsum rock or shale caprock, and stable trapping and preservation conditions are the key factors for accumulation of ultra-deep oil and gas. We propose three favorable exploration directions, i.e. the areas around intracratonic rift and intracratonic depression, and craton margin.

Quantitative analysis of geological attributes for reservoir heterogeneity assessment in carbonate sequences; a case from Permian–Triassic reservoirs of the Persian Gulf

This study integrates quantitative analysis of reservoir properties with geological parameters to discriminate the reservoir heterogeneities in the Permian–Triassic sequences of the Persian Gulf. To achieve this goal, geological analysis of core samples and thin sections were integrated with petrophysical logs and porosity–permeability data. For quantitative assessment of reservoir heterogeneity, Coefficient of Variation (CV), Dykstra-Parsons Coefficient (VDP) and Lorenz Coefficient (VLC) were used along with the concept of Hydraulic Flow Units (HFU). Depositional facies, diagenetic alterations and sequence stratigraphic framework of the reservoir were used as a basis for the interpretation of geological heterogeneities. Accordingly, 14 microfacies types and 5 facies belts have been defined and indicate a low-angle (homoclinal) ramp model. Complex diagenesis history of these sequences includes marine, hypersaline and shallow to deep burial diagenesis that considerably altered the primary distribution of reservoir properties. A comparison of tested methods revealed that the CV is the best approach for quantification of heterogeneity in targeted reservoir. Consequently, the K4 and K2 are proposed as the most homogeneous units with a dominance of grain-supported facies of shoal setting, intense effects of hypersaline dissolution and lesser effect of compaction and cementation. The K3 and K1 units show high levels of heterogeneity resulting from the combined facies changes and diagenetic alterations (dolomitization, cementation and compaction). Finally, sequence stratigraphic framework of reservoir heterogeneities is discussed for all reservoir units and can be used as a basis for ongoing reservoir studies of the Permian–Triassic sequences in the Persian Gulf and the Middle East.

Reservoir forming conditions and exploration potential of Lower Paleozoic carbonate rocks in Gucheng area, Tarim Basin

The Gucheng region is oil and gas accumulation area of Tarim Basin，dominated by beach controlled lithologic oil and gas reservoirs，which has superior oil and gas accumulation conditions and broad exploration prospects. The lower Paleozoic strata in the Gucheng region are the Carboniferous, Ordovician, and Cambrian systems from top to bottom. The Silurian, Devonian, and Permian systems are missing. The lower Ordovician – Cambrian are the main exploration target layers in the area. There are four reservoirs: the Ordovician group, the Yingshan group, the Penglaiba group and the Cambrian group in The Gucheng region, The upper part of the Yijianfang formation and Yingshan formation are limestone reservoir, which is a set of potential reservoir. The lower part of the Yingshan formation, the Penglaiba formation, and the Cambrian formation developed three sets of dolomite reservoirs. The distribution of oil and gas in Gucheng region was mainly found in Yijianfang formation, Yingshan formation, Penglaiba formation and Cambrian of the Ordovician. The Yingshan group and the Hanwu group were the main exploration target layers. Yingshan formation and Cambrian are the main exploration targets, and Yijianfang formation and Penglaiba formation of Ordovician can be used as the concurrent exploration targets. Gucheng region can form two major realistic exploration areas: dolomite reservoir of Ordovician and reef beach of Cambrian. The Ordovician dolomite reservoirs are mainly searched for the inner shoal of the platform edge high-energy facies belt, and the Cambrian system is mainly search for the high-energy reef beach far away from the platform edge facies belt in the north of cherchen fault.

Cambrian Ordovician sedimentary facies and its evolution in Tadong area

In order to clarify the sedimentary development law under the Cambrian Ordovician regional stratigraphic framework in Tadong area, and lay a theoretical foundation for further oil and gas exploration in the study area. The distribution, characteristics and evolution of main sedimentary facies belts of Cambrian Ordovician are studied by means of drilling core observation, cast thin section identification, logging curve feature analysis, seismic profile and well connection profile. The results show that the Cambrian middle lower Ordovician in Tadong area is equivalent to a second-order sequence and can be further divided into 12 thirdorder sequences. Each third-order sequence is mainly composed of transgressive and highstand tracts. Carbonate platform margin beach facies and Reef (mound) beach complex facies are favorable reservoir development facies belts in this area; Under the regional stratigraphic framework, three types of sedimentary facies can be identified in Cambrian Ordovician, and a total of 10 subfacies are developed; The evolution of sedimentary facies is mainly controlled by the rise and fall of sea level, which is characterized by the migration of platform margin facies and the change of platform facies.

Early-Middle Jurassic stepwise deepening in the transitional facies belt between the Adriatic Carbonte Platform Basement and Neo-Tethys open shelf in northeastern Montenegro evidenced by new ammonoid data from the early Late Pliensbachian (Lavinianum Zone)

New ammonoid data prove an early Late Pliensbachian deepening event above the ?Late Hettangian-Sinemurian shallow-subtidal gray-reddish micro-oncoidal-foraminifera grainstone facies and the ?early Pliensbachian deeper-marine micro-oncoidal-crinoidal-ammonoid wacke- to packstone facies. Based on the presence of Fuciniceras lavinianum (Fucini), Lytoceras ovimontanum Geyer and Arieticeratinae gen. indet. from a hardground above the deeper-water micro-oncoidal limestones in the Mihajlovici section (north - eastern Montenegro) a Late Pliensbachian to early Toarcian condensation horizon is proven. The Middle Toarcian ammonoid-bearing horizon also yielded species not known from previous studies: Calliphylloceras capitanii (cATuLLo), Harpoceras subplanatum (oPPeL) and Furloceras aff. chelussii (PAriScH & ViALe), also described in the present paper. These new data prove a stepwise deepening of the depositional area during the early and the Middle Jurassic reflected in detail in four sedimentary members: 1) ?Late Hettangian to Sine - murian/?earliest Pliensbachian open-marine shallow subtital micro-oncoidal limestones; 2) ?early to Late Pliensbachian open-marine condensed limestones with few micro-oncoids and more open-marine influence; 3) Toarcian open-marine condensed red limestones with hardgrounds; and 4) condensed red nodular Bositra Limestone. These four members are separated by hard - rounds representing stratigraphic gaps in deposition. The stepwise deepening during the early-Middle Jurassic follows the general trend of deposition as known in the whole Western Tethys realm above the Late Triassic Dachstein carbonate Platform.

MECO and Alpine orogenesis: Constraints for facies evolution of the Bartonian nummulitic and Solenomeris limestone in the Argentina Valley (Ligurian Alps)

AbstractThe Eocene represents the last greenhouse interval before Present. The maximum warming during the early Eocene was followed by a long‐term cooling trend culminating in the Antarctica glaciation at the base of the Oligocene. Superimposed on this long‐term cooling trend there is a prominent transient warming event known as the middle Eocene climatic optimum (MECO) occurring during the early Bartonian. The carbonate ramp succession cropping out in the Argentina Valley (Maritime Alps) offers new insights on the evolution of shallow water realms during this time interval. This ramp displays two main facies belts, middle and outer ramp. The middle ramp is recorded by larger benthic foraminifer floatstone to rudstone, passing to Solenomeris branches and nodule floatstone to rudstone evolving to branching coralline algal floatstone. The outer ramp is dominated by bioturbated marly wackestone to packstone alternating with larger benthic foraminifer floatstone with a silty matrix. The investigated ramp was affected by continuous dispersion and reworking of the skeletal components as other Tethyan Eocene ramps. During the Eocene, the Alpine foreland was influenced by fine terrigenous input controlling the trophic conditions and promoting seawater stratification and the development of a strong pycnocline, for which many perturbations could propagate as internal waves. The reworking of skeletal components of the ramp has been ascribed to the action of internal waves. The switch of carbonate production from a carbonate factory dominated by larger benthic foraminifera to a factory in which the encrusting foraminifer Solenomeris was the main carbonate producer biota, is indicative of a radical change in palaeoenvironmental conditions affecting the early Bartonian. The acme of Solenomeris often coincided with the crisis of carbonate producers during intervals of an evident deterioration of environmental conditions. In this case the acme is probably related to the adverse conditions linked to the (MECO) warming event. Finally, the drowning of the nummulitic ramp has been caused by light reduction for the photo‐dependent biota due to progressively increasing depth linked to flexural subsidence of the foreland plate, and minor efficiency of the aphotic carbonate factory.

Occurrence of Zoophycos in the Ruteh Formation, Middle Permian (Guadalupian), Central Alborz, Iran: palaeoenvironmental and sequence stratigraphy implications

Zoophycos is widely distributed in the marine strata of the Middle Permian Ruteh For- mation in the Alborz Mountain, Iran. The investigation of the Zoophycos, along with environmental variables is a useful tool for interpretation of the palaeoenvironmental and sequence stratigraphy anal- ysis. The petrographic observations led to the identification of ten facies in four facies belts including tidal flat, lagoon, shoal, and open marine, deposited on a homoclinal ramp. Moreover, two third- order depositional sequences were recognized in response to the sea- level fluctuations within the Ruteh For - mation. Detailed studies of the sequence stratigraphy revealed a relationship between the occurrences of Zoophycos and changes in the hydrodynamic condition in the basin. It appears that Zoophycos has been influenced by the ecological and palaeoenvironmental parameters, such as sedimentation rate, nutrient supply, oxygen, wave base, and substrate in the shallow to deep environments. Based on the sedimentological and ichnological analysis, Zoophycos has been formed with various dimensions, morphology, fillings, and densities together with rising and falling in the sea-level. The trace- maker has followed an opportunistic strategy in the unstable conditions of shallow environments, whereas it has chosen a k-selected strategy in more stable deep environments. Additionally, variability in Zoophycos illustrates, how the trace- maker adopted itself with environmental sequences. This reason, owing to optimal conditions, has caused that the abundance of Zoophycos was high in the Transgres- sive System Tracts (TST). Evidence shows that the response of Zoophycos to the ecological properties of the environment usually has deposit- feeder and chemosymbiosis behaviours.

Characteristics and main controlling factors of reservoirs in the fourth member of Sinian Dengying Formation in Yuanba and its peripheral area, northeastern Sichuan Basin, SW China

Abstract Based on the abundant outcrops and 3D seismic data in Yuanba and its surrounding areas in northeastern Sichuan Basin, the characteristics, main controlling factors and development regularities of reservoirs in the fourth member of Sinian Dengying Formation (shortened as Deng 4 Member) were analyzed comprehensively. The results show that the reservoirs were mainly developed in microbial dolomite associated with the microbial mound and dolarenite associated with grain beach, and had intergranular dissolution pores, inter-frame dissolution pores and karst caves as main storage space. The reservoirs were large in scale and strong in heterogeneity. The analysis of the reservoir genetic mechanisms shows that the reservoirs in the fourth member of Dengying Formation were the products of karstification on mound-beach facies deposited in the second episode of Tongwan movement. The karstification had the marks of eogenetic karstification such as granophyric karst system and half-dissociation zone. The reservoirs were controlled by the sedimentary facies belt, the mound beach facies was the decisive factor in reservoir development. Further analysis indicates that the Yuanba area had a geological background of well-developed platform margin mound beach facies, so mound beach bodies can be taken as a favorable exploration target.

Using characteristics of pressure-temperature induced permeability variation of typical carbonate rock to determine its performance as reservoir or cap rock

The current research on carbonate rock often focuses on the storage space for oil and gas, and exploration of carbonate rock in petroliferous basins also mainly searches for high-quality reservoir rock for hydrocarbons. However, the importance of effective carbonate reservoir-cap rock configuration is increasingly highlighted, as hydrocarbon exploration advances towards deeper and older strata. Exploration practice shows that the favorable facies belt of carbonate rock, though deeply buried, still develops highly desirable reservoir rock with high porosity and permeability. Therefore, the presence of an effective carbonate cap rock collaborating with this reservoir rock becomes key to hydrocarbon exploration in deep carbonate rock. This research, for the first time, systematically and thoroughly carries out permeability measurement through the whole triaxial compression process of typical carbonate rock at varied temperatures, and captures the permeability evolution during deformation and failure, affected by temperatures and confining pressures. Test results show that typical dolomite and limestone present varied permeability variation characteristics in a triaxial loading state. Dolomite appears to be more of reservoir rock. It presents higher critical confining pressure for brittle–ductile transition, and is more prone to fracturing. Its permeability grows with temperature elevation. As for limestone, it has better performance as cap rock, which is embodied as the lower critical confining pressure for brittle–ductile transition, high sealing capacity, and permeability reduction with elevated temperatures. To sum up, vertical stacking of dolomite and limestone can form an effective reservoir-cap rock assemblage for hydrocarbon accumulation.

Deep-T-platform responses to the global sea-level fluctuations, Oligocene Asmari and Pabdeh Formations of the Zagros foredeep Kalhur sub-basin, SW Iran

A sequence stratigraphic analysis of the Asmari and Pabdeh Formations (Oligocene) carbonate factory in the Kalhur sub-basin of the Zagros foredeep in SW Iran established the depositional attributes and facies patterns of the abnormally deep-T-platforms in response to global sea-level fluctuations. The paleoecology, paleogeography and hydrodynamic properties affected symbiotic large benthic foraminifers (SLBF) to determine the carbonate generation window and steepened ramp systems building from gravel-producing biota within the oligophotic zone. Four facies belts, containing five lithofacies and ten microfacies like those of C-platforms, were identified from grain associations and standard depositional criteria. Highstand systems tracts formed of uncemented and unconsolidated close-cluster reefs developed in the lower mid-ramp to upper part of the outer ramp and prograded below storm wave base (SWB). Portions of the reefs above SWB were eroded and transported downslope to the distal portion of the outer ramp and the proximal part of the basin as tempestites, created by periodic storms. In the subsequent sea-level fall, these platforms were eroded during periodic storms, shedding large volumes of calciturbidites, just as they do in C-platforms. The Rupelian succession consists of three third-order sequences, namely Ru. 1, Ru. 2 and Ru. 3. Further comparison of sea-level and onlap curves in the study area with global and Arabian platform sea-level and onlap curves, provides an exact chronostratigraphic framework, revealing a close correlation with eustatic events in the Zagros foredeep.

Reef response to sea-level and environmental changes in the Central South Pacific over the past 6000 years

Geological records of coastal system evolution during past higher and/or rising sea levels provide an important baseline for developing projections regarding the response of modern coastal systems to future sea-level rise. The mid-late Holocene corresponds to the most recent window into natural variability prior to the Anthropocene and involves slow-rate and low-amplitude sea-level changes that were mostly governed by a limited glacio-eustatic contribution, most likely sourced from Antarctica, and ‘glacial isostatic adjustment’ processes. This paper documents in unprecedented detail the response of coral reefs and coastal systems to changing accommodation space in relation to mid-late Holocene sea-level changes in French Polynesia. The sea-level curve that underpins this study has a global significance and documents a single short-lived sea-level highstand between 4.10 and 3.40 kyr BP. The amplitude of the highstand is less than one metre, within the range of the predicted sea level at the end of the current century. The reported relative sea-level changes are characterized by slow rates ranging from a few tens of millimetres per year to up to 2.5 mm/yr and by significant sea-level stability (stillstands) lasting more than a century and up to 250 years, defining a step-like pattern. Sea-level variability probably driven by climatic oscillations on interannual to millennial time scales is evidenced during the entire time window. The detailed reconstruction of reef development over the last 6000 years brings valuable information regarding coral reef dynamics and coastal processes during periods of higher sea level and wave energy regimes. The persistence of stable and optimal depositional environments over the last 6000 years is demonstrated by the constant overall composition and diversity of reef communities and the almost continuous development of coral microatolls. The facies distribution as well as the lateral extension and shift of facies belts have been governed by variations in accommodation space, which are controlled by relative sea-level changes and antecedent topography. The widespread development of mid-late Holocene reef deposits in coastal areas suggests that they have played a prominent role in global processes related to the formation and shaping of modern islands. • Coral reef responses to slow-rate, low-amplitude, high-frequency sea-level changes. • Continuous reef development over the last 6000 years in optimal conditions. • Mid-late Holocene coral assemblages similar to their modern counterparts. • Facies distribution governed by variations in accommodation space. • Prominent role played by reef deposits in the formation and shaping of modern islands.

Carbonate platform evolution of the Tirgan formation during Early Cretaceous (Urgonian) in the eastern Kopet-Dagh Basin, northeast Iran: depositional environment and sequence stratigraphic significance

Tirgan formation of the Kopet-Dagh Basin (northeast Iran) represents one of the Urgonian carbonate platforms that were deposited during the Early Cretaceous time in the northern Alpine Tethys and deformed during the Alpine orogeny. In this study, six stratigraphic sections of the shallow-water platform sediments (Tirgan formation) were measured based on microfacies and fauna abundance. Detail study of petrography, fossil content, and sedimentary structures led to the identification of fifteen microfacies belonging to four facies belts including open marine, shoal, protected lagoon, and tidal flat. The sediments of the Tirgan formation exhibit calcareous green algae, abundant ooids, oysters, bryozoans, and crinoids in inner and middle platform ramp facies and planktonic bivalves and sponge spicules in outer-platform facies. Furthermore, the absence of basinal deposits and lack of evaporate evidence point to deposition under warm-temperate and humid climate conditions. Sequence stratigraphy analysis of Tirgan formation led to distinguish a single depositional sequence in all of the sections which are composed of transgressive and highstand systems tracts with sequence boundaries of type II (SB2). The lowermost lower Aptian Tirgan sequence in the study area relatively shows a similar trend in comparison with the global curve. This basin was deepened over time so that shaly and marly sediments of Sarcheshmeh formation were placed over Tirgan conformably and may suggest a drowning event that was likely related to unusual global warming. Last, this study contributes to the better understanding of the high distribution of facies assemblages in the Urgonian carbonate platforms.

Porosity system analysis and its relation to the permeability of the reservoir zones in the Upper Dalan Member and Kangan Formation: A case study, one of the gas fields in the Zagros Coastal Fars, SW Iran

The studied gas field is located in the Coastal Fars structural zone of the folded Zagros, in the west of Hormozgan Province and on the Gavbandi highland. The Dehram Group forms gas reservoirs in this field. The results of macroscopic and microscopic studies and routine core analysis of the Upper Dalan Member and Kangan Formation show that porosity with an average of 14.51% in the Kangan Formation is more than Upper Dalan Member with an average of 8.29% (certain intervals of the well cores); however, the Upper Dalan Member with an average of 10.8 md (10.8 × × 10−3 μm2) has higher permeability than Kangan Formation with an average of 3.54 md (3.54 × × 10−3 μm2). The Grain-supported facies (in accordance with Ka-2 reservoir zone) of the Kangan Formation has the best reservoir quality with an average of 10.7% effective porosity and 43.3% total water saturation; while in the Upper Dalan Member, mud-supported facies (in accordance with UDL-5 reservoir zone) has the best reservoir quality with an average of 10.2% effective porosity, and 36.7% total water saturation where the dolomitization process has been the most important factor for increasing the reservoir quality in these facies. The porosity type of the Kangan reservoir (oolitic grainstone of shoal facies belt in Ka-2 reservoir zone) is isolated moldic and vuggy those generates high porosity and low permeability values in this formation but in some intervals, the isolated molds and vugs are connected by channel and fracture; therefore increased the value of permeability. The tidal flat facies belt of Upper Dalan member has high porosity and permeability and good reservoir quality in the UDL-2, UDL-5 and UDL-6 reservoir zones, with respect to the tidal flat facies of the Kangan Formation, which is related to dolomitization processes that generate intercrystalline porosity type. Finally, both the depositional environment and diagenetic processes controlled the distribution characteristics of reservoir quality, as seen in the relation between porosity and permeability.

Microfacies analysis, diagenetic overprints, geochemistry, and reservoir quality of the Jurassic Samanasuk Formation at the Kahi Section, Nizampur Basin, NW Himalayas, Pakistan

In Pakistan, carbonate rocks of Jurassic age are present entirely throughout the Indus Basin. The Jurassic carbonate rocks present in the Attock-Cherat requires a more detailed investigation as of recent discovery of gas in Jurassic carbonate rocks in the lower Indus basin. The evaluation of these Jurassic carbonate units in the Nizampur Basin of Attock-Cherat ranges should integrate the aspects of the depositional sediments, diagenetic history, and geochemistry to understand the reservoir behavior. In this study, the Samanasuk Formation of 90 m was sampled and measured in detail, and samples were collected with ~ 0.3 m interval and some samples were collected where reckoned necessary. Petrographic and microfacies analysis were conducted on about 100 samples; moreover, XRD analysis were performed. The present study aims to determine the depositional facies, diagenetic processes, and geochemical elements of this carbonate succession of the in an effort to explore their effect on reservoir quality. The Samanasuk Formation comprises eight microfacies assigning three facies belts including peritidal, lagoon, and shoal of a carbonate ramp. The recorded diagenetic processes include dolomitization, compaction, micritization, neomorphism, dissolution, and cementation in which dolomitization played an important role in enhancing the reservoir quality. In relation of their impacts on reservoir properties, the grainstone facies associated with peritidal facies and dolomudstone facie associated with carbonate sand and shoal shows the greatest reservoir quality, whereas lagoonal facies has the lowest reservoir quality. This study represents an approach to use the depositional facies, diagenetic alterations, and geochemical framework of carbonate succession in the reservoir characterization.

Sequence stratigraphy of Albian–Campanian carbonate deposits (Sarvak and Ilam formations) in Shiraz area, Fars, SW Iran

Bangestan group in SW Iran consists of Sarvak and Ilam formations. The Sarvak and Ilam formations comprise a thick succession of carbonate rocks in Zagros basin of Southern Iran. These formations form one of the main hydrocarbon reservoirs in central Fars, Shiraz area. This study presents a facies analysis, sedimentary environments and sequence stratigraphy. To study petrography, microfacies, sedimentary environments and sequence stratigraphy of Sarvak and Ilam formations one stratigraphic section has been chosen and studied (MajAbad section). Facies analysis led to recognition of 13 microfacies and 2 lithofacies. Study of microfacies components such as allochems and ortochems, relative sea level change and sequence stratigraphy evidences show these facies were deposited in carbonate ramp sedimentary environment in 4 facies belt zones: open marine (A), Shoal (B), Lagoon (C) and Tidal flat (D) during Middle Creataceous. Petrography, microfacies analysis and sedimentary environment of Sarvak and Ilam formations show these formations in study area consist of 5 3 order depositional sequences. Sequence 1 to sequence 4 related to Sarvak Formation and sequence 5 covered of Ilam formation. These depositional sequences have been seperated by both type-1 and type-2 sequence boundaries. Sequence stratigraphy evidences show that the Sarvak and Ilam Formations constitute five separate depositional sequences which are separated by a thin palaeosol, representing a type-one and type -two sequence boundary which can be correlated with global curves of relative sea-level.

Isolated carbonate platform reservoir multiple grouped discrete fracture network modelling

The fracture distribution on isolated carbonate platform is complicate congregation, it is necessary to accurately characterize fracture for oilfield development purpose. Based on the imaging logging fracture interpretation, this study analyzed the fracture grouping condition by sedimentary facies belt, also evaluated corresponding fracture intensity and orientation parameters; the fracture orientation trend for azimuth continuously changes fracture group can be established using the fracture group azimuth parameter; the intensity property calculation of each fracture group was carried out by Sequential Gaussian Simulation method under the seismic curvature volume control; each group’s discrete fracture network calculated from fracture intensity property and the analyzed fracture parameters accurately described the fracture distribution and orientation, the all groups together upscaled fracture property model shows permeability which consistent with with geological concept. This study shows: fracture features significant differentiated in each sedimentary facies belt on isolated carbonate platform, reservoir fracture can be characterized reasonably and accurately by azimuth trend control, thus lay the foundation for the reservoir simulation.

Supradetachment to rift basin transition recorded in continental to marine deposition; Paleogene Bandar Jissah Basin, NE Oman

AbstractA transition from supradetachment to rift basin signature is recorded in the ~1,500 m thick succession of continental to shallow marine conglomerates, mixed carbonate‐siliciclastic shallow marine sediments and carbonate ramp deposits preserved in the Bandar Jissah Basin, located southeast of Muscat in the Sultanate of Oman. During deposition, isostatically‐driven uplift rotated the underlying Banurama Detachment and basin fill ~45° before both were cut by the steep Wadi Kabir Fault as the basin progressed to a rift‐style bathymetry that controlled sedimentary facies belts and growth packages. The upper Paleocene to lower Eocene Jafnayn Formation was deposited in a supradetachment basin controlled by the Banurama Detachment. Alluvial fan conglomerates sourced from the Semail Ophiolite and the Saih Hatat window overlie the ophiolitic substrate and display sedimentary transport directions parallel to tectonic transport in the Banurama Detachment. The continental strata grade into braidplain, mouth bar, shoreface and carbonate ramp deposits. Subsequent detachment‐related folding of the basin during deposition of the Eocene Rusayl and lower Seeb formations marks the early transition towards a rift‐style basin setting. The folding, which caused drainage diversion and is affiliated with sedimentary growth packages, coincided with uplift‐isostasy as the Banurama Detachment was abandoned and the steeper Marina, Yiti Beach and Wadi Kabir faults were activated. The upper Seeb Formation records the late transition to rift‐style basin phase, with fault‐controlled sedimentary growth packages and facies distributions. A predominance of carbonates over siliciclastic sediments resulted from increasing near‐fault accommodation, complemented by reduced sedimentary input from upland catchments. Hence, facies distributions in the Bandar Jissah Basin reflect the progression from detachment to rift‐style tectonics, adding to the understanding of post‐orogenic extensional basin systems.