Carbonate Karst Reservoirs Research Articles

Fault system dynamics and their impact on ordovician carbonate karst reservoirs: Outcrop analogs and 3D seismic analysis in the Tabei region, Tarim Basin, NW China

The Tabei Uplift in the Tarim Basin of NW China represents an area with significant hydrocarbon exploration potential. This paper describes the effects of multi-phase faults on structural and hydrocarbon systems in the Tabei Uplift utilizing high-quality 3-D seismic, geological outcrops and drilling and production data. The interpretation and analysis of 10 fault systems from the Cambrian through the Cretaceous, shows that the deep, middle and shallow faults developed in four stages: the Early Caledonian, Mid-Late Caledonian, Hercynian and Himalayan. Specifically, the Early Caledonian fault system is favorable for the hydrocarbon expulsion of Cambrian source rocks, while Mid-Late Caledonian strike-slip faults facilitate hydrocarbon migration and accumulation. Hercynian and Himalayan strike-slip faults were formed as a result of the reactivation of the Mid-Late Caledonian strike-slip faults and have a destructive effect on the older oil reservoirs. Similarly, Permian volcanic activity also caused varying degrees of damage to Ordovician reservoirs. Oil production along fault zones more than 100 km show that the complex fracture-vuggy (FV) structure is more likely to trap oil and gas when it is damaged than the simple FV structure. Fault-controlled, fracture-vuggy carbonate reservoirs (FVCRs) were classified into six grades, which correlate to the productivity of oil wells in the region. Using this classification system, several drilling locations were selected and resulted in 80% success rate. This classification provides a novel approach for assessing the interplay between fault activity and reservoir formation, enhancing the prediction of hydrocarbon potential in varying strike-slip fault zones. This analysis not only refines our understanding of the region's geodynamic history but also aids in optimizing exploration and development strategies.

Residual carbonate karst reservoir reconstructed by karst planation: A case study of Ordovician paleokarst reservoir characterization in Ordos Basin, North China

Based on the global typical karst characteristics reported in recent decades and the latest petroleum and gas exploration results, it is found that the characteristics of the karst reservoirs in the Ordos Basin are dramatically different from those of traditional karst reservoirs, indicating that the main contribution to the formation of the karst reservoirs in the region may not have been karst dissolution. The previous view of the formation mechanism of karst reservoirs suggests that we should think about the formation and development processes of Ordovician karst and the reservoir pores in the Ordos Basin from another perspective. In this investigation of reservoir formation, karstology, geography, sedimentology, and reservoir geology were all utilized. On the basis of the initial understanding of the control of large-scale landforms on the sediments in sedimentary basins, this study focuses more on the dynamic spatial-temporal relationships between the evolutions of the landforms, karsts, and reservoirs. By studying the petrological characteristics, pore filling, karst zonation, and planation model of the weathering crust reservoir in the study area, it has been determined that the primary reservoir space was formed in advance rather than in the supergene dissolution period. A gypsum-containing dolomite flat served as the material foundation for reservoir construction and later karst planation transformation. The penecontemporaneous dissolution dominated the spatial development of the early dolomite reservoir. The traditional karst cycle is actually caused by the erosion of the bottom of the karst floor on the double-layer leveling surface. The karst planation during the Caledonian period played a discontinuous and destructive filling role throughout the entire reservoir zone by changing the diagenetic environment, the main contribution of which was communication between the pores. Therefore, this weathered crust reservoir is actually a residual reservoir controlled by karst planation rather than a traditional karst reservoir. The “sweet spots” are primarily dispersed in the upper portion of the karst slope and the top of the karst residual hillock in the gypsum-containing dolomitic flat environment.

Formation mechanisms and resource potential of carbonate geothermal reservoirs in the Beijing-Tianjin-Hebei plain

Investigating the formation mechanisms of carbonate geothermal reservoirs is of theoretical and practical significance for summarizing the formation pattern of geothermal resources and further guiding their effective exploitation. The Beijing-Tianjin-Hebei Plain (BTHP), predominantly located within the Jizhong Depression and Cangxian Uplift in the Bohai Bay Basin, serves as the primary region for geothermal exploitation and utilization in China. More than 1500 geothermal wells have been drilled therein, with water temperature at the wellhead ranging from 55 to 110 °C, single-well flow rate ranging between 80 and 120 m3/h, and cumulative heating area exceeding 100 × 106 m3. However, the exploration and research in the region remain limited overall. As per the previous geothermal and petroleum exploration results and the latest geothermal drilling data, this study comprehensively evaluated the geothermal resources of karst geothermal reservoirs. The results show that two suites of carbonate karst reservoirs, namely the Jxw Formation and the Ordovician strata, have primarily developed in the BTHP, and their formation and evolution can be divided into four stages: the Mesoproterozoic-Early Paleozoic stage with carbonate sedimentation and the development of interlayer karst, the Late Paleozoic stage with the development of direct sedimentary cover, the Mesozoic stage with compressional uplifting and development of buried hill karst, and the Cenozoic stage with regional cover deposition and the modification and finalization of karst geothermal reservoirs. Accordingly, the porosity evolution history of the geothermal reservoirs is composed of three stages, namely a significant decrease followed by a minor increase, a gradual decline, and then a small fluctuation from increase to decrease before slowly rising again. The karstification in geothermal reservoirs can be summarized into quasi-syngenetic karstification, epigenetic karstification, and burial karstification, which can be subdivided into seven subcategories. The carbonate geothermal reservoirs in the study area boast total geothermal resources of 53.821 × 109 GJ, or 184.155 × 109 t of standard coal equivalent (tce), and the annual exploitable geothermal resources in the urban area can heat an area of (400‒500) × 106 m3, indicating great potential of geothermal exploitation.

Structural attributes, evolution and petroleum geological significances of the Tongnan negative structure in the central Sichuan Basin, SW China

The Tongnan secondary negative structure in central Sichuan Basin has controls and influences on the structural framework and petroleum geological conditions in the Gaoshiti-Moxi area. To clarify the controls and influences, the deformation characteristics, structural attributes and evolution process of the Tongnan negative structure were investigated through a series of qualitative and quantitative methods such as balanced profile restoration, area-depth-strain (ADS) analysis, and structural geometric forward numerical simulation, after comprehensive structural interpretation of high-precision 3D seismic data. The results are obtained in three aspects. First, above and below the P/AnP (Permian/pre-Permian) unconformity, the Tongnan negative structure demonstrates vertical differential structural deformation. It experiences two stages of structural stacking and reworking: extensional depression (from the Sinian Dengying Formation to the Permian), and compressional syncline deformation (after the Jurassic). The multi-phase trishear deformation of the preexisting deep normal faults dominated the extensional depression. The primary depression episodes occurred in the periods from the end of Late Proterozoic to the deposition of the 1st–2nd members of the Dengying Formation, and from the deposition of Lower Cambrian Longwangmiao Formation–Middle–Upper Cambrian until the Ordovician. Second, the multi-stage evolution process of the Tongnan negative structure controlled the oil and gas migration and adjustment and present-day differential gas and water distribution between the Tongnan negative structure and the Gaoshiti and Moxi-Longnüsi structural highs. Third, the Ordovician, which is limitedly distributed in the Tongnan negative structure and is truncated by the P/AnP unconformity on the top, has basic geological conditions for the formation of weathering karst carbonate reservoirs. It is a new petroleum target deserving attention.

Analytical model for transient pressure analysis in a horizontal well intercepting with multiple faults in karst carbonate reservoirs

In recent years, the Shunbei karst-carbonate reservoirs becomes a huge productivity oilfield, which produced over one million tonnes crude oil annually. However, there are difficulties in understanding production contribution of each fault-karst branch in reservoirs, which significantly impacts the efficient development. Multibranched fault-karst reservoirs in the Shunbei have an obvious tree-shaped geostructure, in which the natural fractures and eroded cave develop along multiple large-scale faults. The existing models for pressure transient analysis (PTA) were mainly established for fracture-cave reservoirs only with single fault, which cannot be applicable to characterize the multibranched fault-karst reservoir. To fill this gap, a novel analytical PTA model for horizontal commingled production well in the multibranched fault-karst reservoir was established to describe pressure response and identify flow regimes. First, our model includes the Darcy flow with the fluid compressibility effect in fracture region, and the large-scale vertical storage flow in cave region as well as the horizontal laminar flow in the horizontal wellbore. Then, the accuracy of this PTA model is verified by comparing it with the existing single branch fault-karst pressure model. Further, we applied the model to analyze the Shunbei oilfield case data. Last, the effect of boundary type, fluid compressibility effect, fracture physical properties, and cave spatial distribution on the pressure response are discussed in detail. The sensitivity analysis results show (a) the cave storage flow regime exhibits an obvious unit-slope-line on pressure derivative curve, at the time the skin transient flow constitutes a V-shape characteristic. (b) The number of fracture-cave branches can be directly obtained by counting the number of V-shaped appearances on the pressure derivative curve. (c) The fluid compressibility effect leads to an upward trend on the pressure and its derivative, reservoir engineers should be cautious to explain that characteristic as a closed boundary effect. (d) The cave volume and cave position control the timing of the V-shape occurring. As the cave volume increases, the linear flow regime lasts longer and the V-shaped feature becomes apparent. With the cave distance and cave depth increasing, the V-shape characteristic comes later. This work can provide technical support for accurate characterization of multibranched fault-karst reservoirs, and give a type curve analysis method for rapidly diagnosing the spatial location of each karst cavity by analyzing bottom-hole pressure.

Applicability Analysis of Pre-Stack Inversion in Carbonate Karst Reservoir

Although pre-stack inversion has been carried out on reservoir prediction, few studies have focused on the application of pre-stack for seismic inversion in fractured-cavity carbonate reservoirs. In carbonate rock, complicated combinations and fluid predictions in karst caves are remain unclear. Post-stack methods are commonly used to predict the position, size, and fillings of caves, but pre-stack inversion is seldom applied in carbonate karst reservoirs. This paper proposes a pre-stack inversion method for forward modeling data and oil survey seismic data, using both points to indicate the application of pre-stack inversion in karst caves. Considering influence of cave size, depth, and filler on prediction, three sets of models (different caves volume; different fillings velocity of caves; complicated combination of caves) are employed and inverted by pre-stack inversion. We analyze the pre-stack results to depict Ordovician oil bearing and characterize caves. Geological model parameters came from actual data of the Tahe oilfield, and seismic data were synthesized from geological models based on full-wave equation forward simulation. Moreover, a case study of pre-stack inversion from the Tahe area was employed. The study shows that, from both the forward modeling and the oil seismic data points of view, pre-stack inversion is applicable to carbonate karst reservoirs.

Structural Characteristics and their Significance for Hydrocarbon Accumulation in the Northern Slope of the Central Sichuan Paleo‐uplift

AbstractBased on 2D and 3D seismic data, the latest drilling data and field outcrop data of the northern slope of the Central Sichuan paleo‐uplift, the structural analysis method is used to analyze unconformity development characteristics and fault characteristics during the key structural transformation period, discussing the influence of the structural characteristics on the hydrocarbon accumulation of deep carbonate rocks. The results show that: (1) The two key unconformities of the Tongwan and Caledonian periods were primarily developed in deep carbonate rocks. Firstly, Tongwan's unconformities are characterized by regional disconformities between the second and third members of the Dengying Formation, the top formation of the Sinian and the lower Cambrian, strips of which zigzag through the north and south sides of the study area. Secondly, the Caledonian unconformity is characterized by a regional unconformable contact between the lower Permian and the ower Paleozoic strata. From NE to SW, the age of the strata, which were subject to erosion, changes from new to old, the denudation distribution showing as a nose‐shaped structure which inclines towards the ENE. (2) Boundary fault and transtensional strike‐slip faults developed in the Sinian to Paleozoic strata. In profile, there are three types of structural styles: steep and erect, flower structures, ‘Y’ and reversed ‘Y’ type faults. In plane view, the Sinian developed extensional boundary faults extending in an almost NS direction, strike‐slip faults developing and extending linearly in approximately EW, WNW and NE strikes in the Cambrian, with characteristically more in the south and less in the north. (3) The faults in the northern slope show obvious zonal deformations in transverse view as well as significant stages and stratified activity in a longitudinal direction. Among them, the activity of faults in the Sinian was the strongest, followed by the activity in the Cambrian period, the activity intensity of faults in the Permian period being the weakest. This fault activity can be divided into four periods: Sinian, Cambrian‐Permian, the early Indosinian period and the late Indosinian‐Himalayan period, the transtensional strike‐slip faults being the products of oblique extensions of pre‐existing weak zones in the Xingkai and Emei taphrogenesis, with a particular inheritance in the main faults. (4) Combined with hydrocarbon accumulation factors, it is considered that the epigenetic karstification of the Tongwan and Caledonian unconformities in the northern slope controlled the formation and distribution of carbonate karst reservoirs over a large area, also acting as a good pathway for oil and gas migration. The extensional faults developed at the margin of the NS trending rift, controlling the sag‐platform sedimentary pattern in the Dengying Formation of the Sinian. Strike‐slip faults in NE, WNW and ENE directions may control the micro‐geomorphological pattern inside the platform and intensify the differential distribution of grain beach facies. The multi‐stage hereditary activity of strike‐slip faults not only improved the porosity and permeability of the reservoirs, but also acted as the main channel of oil and gas migration, providing favorable conditions for the development of the current multi‐layer gas‐bearing scenario in the northern slope of the Central Sichuan Basin.

Analytical Model for Transient Pressure Analysis in a Horizontal Well Intercepting with Multiple Faults in Karst Carbonate Reservoirs

Analytical Model for Transient Pressure Analysis in a Horizontal Well Intercepting with Multiple Faults in Karst Carbonate Reservoirs

Residual Carbonate Karst Reservoir Reconstructed by Karst Planation: A Case Study of the Ordovician Paleokarst Reservoir in the Ordos Basin

Residual Carbonate Karst Reservoir Reconstructed by Karst Planation: A Case Study of the Ordovician Paleokarst Reservoir in the Ordos Basin

Calculating the Eroded Thickness Corresponding to a Short-Term Tectonic Uplift with Milankovitch Theory: A Case Study of the Middle Permian Maokou Formation in Southeastern Sichuan Basin, Southwest China.

The Middle Permian Maokou Formation in the southeastern Sichuan Basin is a typical carbonate karst reservoir. At the end of the Middle Permian, a short-term tectonic uplift (Tungwu movement) occurred in the upper Yangtze region, causing the formation of dissolved fissures and holes. To determine the location of the high-quality reservoir, this paper calculated the eroded thickness using the Milankovitch theory. Based on the gamma logging data of the six wells in the southeastern Sichuan Basin, the dominant frequency and the astronomical time scale were evaluated via frequency spectrum analysis, continuous wavelet transform, and empirical mode decomposition. In addition, we analyzed the relationship between Fischer curve characteristics and the variation of lithology. Last, four methods were used to calculate the eroded thickness, and the rationality was analyzed. Consequently, we identified four levels of Milankovitch cycles, i.e., middle eccentricity (e2), short eccentricity (e3), long obliquity (o1), and short obliquity (o2). Also, the Fischer curves of the six wells were divided into two forms related to local structural uplift. The residual strata of the Maokou Formation comprised three complete third-order cycles, and the boundaries were the 15th, 34th, and 54th e3 cycles. The deposition rate of bioclastic limestone was the lowest (2.12–5.36 cm/ka with an average of 3.30 cm/ka), whereas the deposition rate of argillaceous limestone was the largest (2.27–5.25 cm/ka with an average of 4.09 cm/ka). Among the four methods, the missing formation deposition rate method exhibited the most precise calculation results, while that of the seismic data method was relatively low. Generally, the eroded thickness of the Maokou Formation in southeastern Sichuan was in the range of 0–140 m, i.e., the eroded thickness in the west and south of X14 was relatively large (>100 m), while the area north of LS1 experienced the weakest denudation (eroded thickness < 40 m).

Types and controlling factors of ordovician paleokarst carbonate reservoirs in the southeastern Ordos Basin, China

With continuous improvements in oil and gas exploration technology and the increasing demand for oil and gas resources, the focus of oil and gas exploration has shifted from deep to ultradeep reservoirs. This study targeted the carbonate karst reservoirs of the Ordovician Ma5 Member in the southeastern Ordos Basin. Based on the analysis of cores, thin sections, cathodoluminescence, scanning electron microscopy (SEM), carbon and oxygen isotopes and trace elements, we discuss the development characteristics, types and influencing factors of paleokarst reservoirs. Petrology and geochemistry show three kinds of karstification in the study area: quasi-contemporaneous karst, epigenetic karst and deep burial karst. The quasi-contemporaneous and epigenetic paleokarsts were mainly controlled by freshwater, where the soluble components of the strata were preferentially dissolved. The deep burial karstification was dominated by the reaction product H2S from thermochemical sulfate reduction (TSR) between sulfate minerals and organic matter, during which hydrothermal fluid provided the required activation energy for the reaction. The observations of cores and thin sections show that the paleokarst reservoirs are composed of different kinds of dissolution pores and fractures and cemented by multistage calcite and siliceous cements. Influenced by freshwater leaching and hydrothermal fluid, the three paleokarst processes led to negative anomalies in δ18O and δ13C, while the low δCe, low δEu and rare earth element (REE) partition modes confirm that the intensity of freshwater leaching was stronger than that of hydrothermal fluid. The controlling factors of the paleokarst reservoirs included acidic fluids, paleogeomorphology and paleostructure in each diagenetic stage. According to the structural background and core characteristics, we establish a development model and spatial evolution model of the paleokarst reservoirs in the southeastern Ordos Basin.

Architectural characterization of Ordovician fault-controlled paleokarst carbonate reservoirs, Tahe oilfield, China

Fault-controlled karst carbonate reservoirs are one of the most important reservoir types in the Tahe oilfield of the Tarim Basin. These reservoirs have a large oil reserve and belong to a strongly reconstructed reservoir type with a highly heterogeneous distribution of pores and fractures. This study characterizes a fault-controlled karst reservoir by using integrated methods, including outcrops, well logging, structure interpretation, seismic inversion, and statistical geomodeling. We have established a fault-/fracture-controlling karstic geologic model and classified the internal architectural elements so that we adopted an origin-controlled hierarchical geomodeling strategy based on the fault-controlling characteristics. The results determined that large strike-slip faults provide an important tectonic framework and that its derived fractures act as important channels and spaces for dissolution. Flower structure fault zones and the associated fractures are the main range of karst development, within which a high stress is concentrated during the strike-slip shear process with a high-density fracture development. This is the highly developed karst reservoir, which mainly is concentrated along large faults. The coexistence of fractures and karst dissolution has resulted in a complicated reservoir architecture (karst architecture), which can be classified into four types: (1) large caverns, (2) small caverns and vugs, (3) fractured zones, and (4) matrix (tight limestone). Controlled by the degree of dissolution, the karst architecture is quite different from the sedimentary facies. Large caverns are formed under the strongest degree of dissolution and are the most favorable reservoir type. Small caves and vugs are created under relatively strong dissolution; they are distributed outside large caves and also can act as favorable reservoirs. The fractured zones are not necessarily affected by strong dissolution but have high conductivity and act as important channels for fluid movement. The carbonate matrix is less reconstructed. The architecture development model of the fault-controlled karst carbonate reservoir presented a tree system, within which the karst reservoir caves are connected by the fractures and faults similar to fruits and trunks. The new geomodeling method revealed the constraining characteristics of faults, seismic attributes, and hierarchical architectural elements. Furthermore, we also have built a 3D model of the Tuoputai unit in the Tahe oilfield to show the robustness of this workflow. This research enables us to better understand the structure of fault-controlled karst reservoirs, and it could provide a specified characterization approach that is considered to be theoretically and practically useful.

Well-controlled dynamic hydrocarbon reserves calculation of fracture\u2013cavity karst carbonate reservoirs based on production data analysis

Fracture–cavity karst carbonate reservoirs have multiple storage space with irregular geometry and various scales, and this caused strong heterogeneity and complex flow characteristics. Accurately calculating the well-controlled dynamic reserves of this kind reservoir effectively is the basis to optimize oil field development plan and making the transformation measures of production well. In order to solve the problem that conventional well-controlled dynamic hydrocarbon reserves calculation method is not suitable for such type reservoirs, we applied a method based on production data analysis. Classification standard of oil well types is established based on the fracture–cavity reservoirs geological static and production dynamic characteristics. Conceptual characteristics of geological model and fluid flow pattern for different types of production wells are assumed. Calculation workflow for well-controlled reserves of fracture–cavity reservoir is established. In the process, some technical key points are proposed to improve accuracy of curve fitting, such as converting the bottom hole pressure from wellhead pressure, correcting the well control range and converting the PVT parameters at the pseudo-steady state. We verified correctness of this method by comparing the calculated results with numerical simulation results of actual production well. This method is well used to Tahe Oil field of Tarim Basin in China. The result shows that the well-controlled reserves quantitative calculation results through this method are in conformity with oil field actual understanding, and the remaining dynamic reserves mainly exist in the drainage area of the wells that drilled different reservoir bodies with good connectivity, especially at the top of large karst caves, and that is the target of further adjusting.

Types of Fluid Alteration and Developing Mechanism of Deep Marine Carbonate Reservoirs

Accurate recognition of the types of alteration fluid and the development mechanisms are important concerns in studying deep marine carbonate reservoirs. Major fluid types, such as seawater, meteoric water, deep burial formation water, hydrothermal fluid, and thermochemical sulfate reduction- (TSR-) derived fluid, were identified based on carbon, oxygen, and strontium isotope compositions of many samples from the Tarim, Sichuan, and Ordos basins in China. Compared with normal marine limestones, seawater calcite cement has similar isotopic compositions. Calcite cement precipitated from meteoric water has extremely light oxygen isotope compositions, and its δ18OV-PDB reaches -18.8‰. Due to the fractionation of oxygen isotopes at high temperatures (101.2~145.6°C), calcite precipitated from deep burial formation water and deep hydrothermal fluid has moderately light oxygen isotope compositions. The TSR process consumes organic matter to produce CO2/CO32-, and the calcite from TSR-derived fluid has very light carbon isotopes (δ18OV-PDB, -18.9‰) due to the incorporation of organic CO2/CO32-. Formation water and TSR-derived fluid generally originate and are confined within the carbonates and are consequently termed endogenous fluids. The 87Sr/86Sr ratios of calcite cements from endogenous fluids are basically the same as those of surrounding carbonates. Meteoric water and hydrothermal fluid originate outside the carbonate strata and are exogenous fluids. The 87Sr/86Sr ratios of calcite cements from exogenous fluids are higher than those of surrounding carbonates, up to 0.710558. For karst carbonate reservoirs developed in tectonic uplift-meteoric water environments, the reservoir spaces of karst caves and fractures occur principally under and near unconformity surfaces and megacrystalline calcite cements occur below the karst zone. In deep fault-hydrothermal fluid environments, high-quality carbonate reservoirs develop downward into ultradeep strata. In deep burial-TSR-derived fluid environments, dissolution porosity can be well preserved for a long geological time due to high CO2 and H2S concentrations.

Improved CRM Model for Inter-Well Connectivity Estimation and Production Optimization: Case Study for Karst Reservoirs

Due to the coexistence of multiple types of reservoir bodies and widely distributed aquifer support in karst carbonate reservoirs, it remains a great challenge to understand the reservoir flow dynamics based on traditional capacitance–resistance (CRM) models and Darcy’s percolation theory. To solve this issue, an improved injector–producer-pair-based CRM model coupling the effect of active aquifer support was first developed and combined with the newly-developed Stochastic Simplex Approximate Gradient (StoSAG) optimization algorithm for accurate inter-well connectivity estimation in a waterflood operation. The improved CRM–StoSAG workflow was further applied for real-time production optimization to find the optimal water injection rate at each control step by maximizing the net present value of production. The case study conducted for a typical karst reservoir indicated that the proposed workflow can provide good insight into complex multi-phase flow behaviors in karst carbonate reservoirs. Low connectivity coefficient and time delay constant most likely refer to active aquifer support through a high-permeable flow channel. Moreover, the injector–producer pair may be interconnected by complex fissure zones when both the connectivity coefficient and time delay constant are relatively large.

Calculation and inducement of lacuna in the Mid-Permian Maokou Fm of Sichuan Basin

Erosion of carbonate strata not only can reflect the karst landform, but also can reveal the characteristics of carbonate karst reservoirs indirectly. In Sichuan Basin, the quantity and root causes of lacuna in the top strata of the Maokou Fm are unclear. In this paper, the features of such stratigraphic lacuna were analyzed qualitatively. Then, in accordance with the spectral transformation of GR logs and the characteristics of cycle in the Maokou Fm penetrated by four typical wells, the Milankovich cycles of the logs were identified. Then they were used for quantitative calculation of erosion quantity and for highlighting the root causes for the stratigraphic lacuna. According to the research results, the Mao-4 Member is residual in the Yibin–Ya'an–Jiangyou area and the Shizhu area, but missing in the other parts of Sichuan Basin with a stratigraphic lacuna intensity gradually increasing from the southern part to the central part and to the northern parts of the basin. Besides, the stratigraphic lacuna of the Maokou Fm varies from 0 to 200 m – for example, 0–60 m in the southwestern and northeastern parts of the basin, and 140–200 m in the southern, central and northern parts of the basin. Moreover, formation denudation occurred at the end of the Maokou Period was induced predominantly by erosion generated as the sea level fell during the glacial age, especially in the northern part of the basin. Also, the Mid-Permian karst landform inherited the sedimentary features of being high in the southwest and low in the northeast. In the areas from the southwestern to central and to the northern Sichuan Basin, the karst landform transits from erosion highlands to karst upper and lower slopes, coinciding well with the sedimentary features of the Wujiaping Period. In conclusion, the Milankovich cycles can be used to calculate the stratigraphic erosion in carbonate formations efficiently and accurately. The technique can be extended to the restoration of stratigraphic erosion in other marine basins.

Characteristics of paleogeomorphology and paleokarstification and the impact on natural gas accumulation: a case study of upper assemblage of Majiagou formation in central Sulige gas field, Ordos Basin, China

The Lower Paleozoic gas reservoirs in central Sulige gas field are mainly accumulated in Ordovician ancient weathering crust karst carbonate reservoirs, and they are especially rich in the upper assemblage of Ordovician Majiagou formation (Ma51–Ma54 sub-members). However, the spatial distribution of gas is unclear yet because of the different denudation degree and uneven development of paleokarstification. This paper mainly focuses on the characteristics of paleogeomorphology and paleokarstification and their impact on natural gas accumulation. Geological stratifying has been done based on the observation of cores and thin sections, and the analysis of drilling, logging and seismic data. The paleogeologic map of pre-carboniferous was depicted according to the ancient weathering crust outcropped strata. Paleogeomorphology reconstruction is based on a comprehensive method combining the moulage method with the residual thickness method. Result indicates that the paleogeomorphology can be divided into western karst highland and eastern karst slope in the area investigated, and the denudation degree of ancient weathering crust in the west is more serious than that in the east. The karst highland can be divided into remnant hill and depression while the karst slope can be divided into gentle hill and shallow depression in detail. Paleokarst section can be vertically divided into epikarstic zone, vertical vadose zone, horizontal phreatic zone and deep slow flow zone based on the analysis of paleokarst identification mark and vertical dissection of single well. By analyzing the relationship between paleokarst characteristics and production data, the study shows that paleogeomorphology and paleokarstification have a significant control on nature gas accumulation and distribution. Gentle hills on karst slope are expected to be the most favorable areas for reservoirs horizontally, horizontal phreatic zone is more advantageous vertically, and second the vertical vadose zone.

Characteristics and origin of the Sinian-Permian fault system and its controls on the formation of paleo-carbonate reservoirs: A case study from Central Paleo-Uplift, Sichuan Basin, China

We have evaluated the existence of good paleo-carbonate reservoirs in fault damage zones with a burial depth exceeding 5800 m in the Central Paleo-Uplift, Sichuan Basin, China. The relationships between fault system and sedimentation, and the formation of the paleo-carbonate reservoirs have been explored, which have long been ignored by previous studies due to the low-quality seismic data and the prevalent assumption of weak tectonic movement. Data from different sources such as newly acquired and processed seismic data, cores, and well-logs are used to study the characteristics and origin of the fault system and their controls on the formation of paleo-carbonate reservoirs. The main findings are as follows: (1) The Sinian-Permian fault system in the study area mainly comprises strike-slip faults with different scales plus a small number of locally developed collapse-related concentric faults. (2) The Sinian-Permian fault system, which usually has normal throw, mainly developed in an extensional stress field and its evolution spanned five stages including the basement-fault formation stage in the Yangtze cycle, extensional dextral strike-slip faults formation stage in the Xingkai cycle, weakly compressional sinistral strike-slip faults formation stage in the Caledonian cycle, extensional faults formation stage in the Hercynian cycle, and compressional transformation stage in the Indosinian-Himalayan cycles. Most faults formed in the Xingkai and Caledonian cycles, whereas the Indosinian-Himalayan cycles had a weak effect on the Sinian-Permian fault system in the study area. (3) Different types of faults have different effects on the sedimentation, formation, and preservation of the paleo-carbonate reservoirs. The synsedimentary faults provide necessary tectonic background for the sedimentation of high-energy facies, whereas the successive faults and coalesced fractures determine the formation, distribution, and preservation of the porous karst carbonate reservoirs. The basement faults controlling the hydrothermal fluids only cause partial filling of the existing pore spaces; thus, most pore spaces in the karst carbonate reservoirs are able to be preserved. Therefore, the fault damage zones within the paleo-carbonate strata produce good reservoirs and important exploration targets.

Forming mechanism of the Ordovician karst carbonate reservoirs on the northern slope of central Tarim Basin

The Ordovician karst carbonate reservoirs on the northern slope of central Tarim Basin are important oil and gas exploration targets in the basin, but their dissolution mechanisms are in controversy. In this paper, based on the integrated study of sedimentation, sequence and reservoir, together with microscopic analysis and macroscopic seismic data analysis, the carbonate karst reservoirs in the study area were divided into three types: dissolved pore-cavity limestone reservoir, pore-cavity dolomite reservoir and fracture-cavity siliceous reservoir, and their forming mechanisms were discussed respectively. Some findings were obtained. First, dissolved pore-cavity limestone reservoirs are distributed in the upper Yingshan Fm and Yijianfang Fm of the Ordovician vertically, while pore-cavity dolomite reservoirs are mainly developed in the Penglai Fm and lower Yingshan Fm of the Ordovician with great thickness. Second, dissolved pore-cavity limestone reservoirs were formed by karstification on the third-order sequence boundary (lowstand tract), while pore-cavity dolomite reservoirs were formed by deep burial dolomitization controlled by karstification on the third-order sequence boundary, both of which are distributed in the highstand tract below the third-order sequence boundary. Third, siliceous reservoirs are developed under the control of faulting, as a result of reworking of deep hydrothermal fluids along faults to the limestone, and the siliceous reservoirs and their hydrothermal solution fracture-cavity systems are distributed near faults. It is further predicted that, in addition to the three types of reservoir above, platform-margin reef-flat reservoirs are developed in the Ordovician on the northern slope of central Tarim Basin.

Strike‐Slip Faults and Their Control on Differential Hydrocarbon Enrichment in Carbonate Karst Reservoirs: A Case Study of Yingshan Formation on Northern Slope of Tazhong Uplift, Tarim Basin

Strike‐Slip Faults and Their Control on Differential Hydrocarbon Enrichment in Carbonate Karst Reservoirs: A Case Study of Yingshan Formation on Northern Slope of Tazhong Uplift, Tarim Basin