Ordovician Carbonate Reservoirs Research Articles

Cenozoic faulting leading to petroleum escape in SW Ordos Basin, China: Evidence from fault-related calcite in situ U-Pb dating, rare earth elements and fluid inclusions

Understanding the link between fault activity and movement to petroleum escape is crucial for assessing the petroleum exploration potential in sedimentary basins. As a significant petroliferous basin in China, the Ordos Basin hosts the world-class uncompartmentalized Jingbian natural gas field within the Ordovician marine carbonate reservoirs. However, the role of the unique exhumation history and the Cenozoic faulting in the formation of such large-scale oil-gas fields remain unclear. Our study combines fault-related calcite U-Pb dating with fluid inclusions analysis offers a unique opportunity to trace petroleum leakage along the the carbonate strata brittle fault zones in the SW Ordos Basin. Calcite samples collected from the carbonate fault zone and across the Ordovician carbonate strata in the SW Ordos Basin reveal clear positive Eu anomaly, slight positive Y anomaly, the lack of positive La and Gd anomalies, and the absence of a negative Ce anomaly. The Y/Ho ratios range from 29 to 36, with an average value of 32, indicating calcite formation in a hydrothermal environment during fault activation. U-Pb dating of these fault-related calcites yields ages of 32.8 ± 3.3 Ma and 23.7 ± 7.4 Ma. Raman spectroscopy of fluid inclusions reveals the presence of CH4 gas, coeval with the fault-related calcite. The homogenization temperatures (Th) of aqueous inclusions coeval with CH4 inclusion from two calcite veins range from 140.0 to 172.4 °C (average Th = 157.8 °C, n = 16) and 127.7–154.3 °C (average Th = 142.2 °C, n = 18). Low temperature thermochronology suggest rapid uplift since 40 Ma, potentially linked to fault activity. The Cenozoic erosion rate was 0.06 mm/y during the ∼9 Myr period from 32.8 to 23.7 Ma in the SW Ordos Basin, suggesting that Cenozoic faulting may have created permeable pathways for petroleum escape. This study provides a temporal link between Cenozoic uplift, faulting, and petroleum loss in SW Ordos Basin.

Paleoclimatic and environmental conditions of the Lower-Middle Ordovician carbonate reservoir in the middle of Tarim Basin (NW China): Constrained by petrographic, geochemical, and carbon–oxygen isotopic characteristics

Breakthroughs have been made recently in the exploration of hydrocarbon in the middle of Tarim Basin, NW China. The complexity of the Lower-Middle Ordovician carbonate reservoir remains enigmatic and hinders the source, developmental mechanism of the calcite fills, and paleo-environmental conditions of the Yingshan and Yijianfang Formations have been unclear. Here we present the related paleoenvironmental settings and diagenetic history, using petrographic, geochemical, and carbon (13C) and oxygen (18O) isotopic characteristics of calcite fracture, cave, and fissure fillings of a Lower-Middle Ordovician carbonate reservoir. The results indicate that the limestone caves dominantly consist of CaO, FeO, and MgO, and minor quantities of TiO2 and MnO. Among the trace elements, Ni reported the highest values (with an average of 1162.16 ppm) and other trace elements such as Ba, V, and Ga occur in minor values (with an average of 71.03, 19.18, and 12.92 ppm) respectively. The enrichment of light rare earth elements (LREEs) over the heavy rare earth elements (HREEs), with positive and negative Ce, and negative Eu anomalies indicate the contexts of oxic and anoxic conditions in seawater and sediments interface. The variation in δ18OPDB values ranges from −4.72 to −14.48‰ (with an average of −10.12‰), indicating a setting of paleo-karstification by atmospheric freshwater dissolution. While the variation in δ13CPDB indicates relatively significant values, ranging between 0.82 to −3.80‰ (with an average of −1.07‰), suggests that the paleo-karstification in this region has experienced episodes of supergene and burial diagenesis. The paleo-temperature average value of 59.1 °C, and the paleo-salinity average (Z-value) of 120.07, indicating a relatively warm paleoclimatic environment during the development of calcite-filling deposits. The results suggest several paleo-environmental conditions when paleokarst occurred: a marine deposition environment, an atmospheric freshwater karstification environment, a shallow burial paleo-karstification environment, and a high-temperature deep-burial environment in Yingshan and Yijianfang Formation in the Shunbei area. This study serves as an analog for similar carbonate systems worldwide, providing valuable insights for understanding paleo-karstification processes in different tectonic and climatic settings. Comparative studies between the Shunbei area, Tarim Basin, and other carbonate platforms contribute to our knowledge of karst evolution and its implications for reservoir development and groundwater resources.

Thermal evolution of deeply buried lower Paleozoic strata and its influence on hydrocarbon accumulation in the Tarim Basin, Northwest China

The Ordovician carbonate reservoirs in the Tarim Basin are hotspots for oil and gas exploration in recent years. However, due to the lack of effective paleo-geothermometers in carbonate formations, thermal history studies have been greatly restricted. In this study, we reconstructed thermal history of Ordovician carbonate intervals in Tarim Basin by applying clumped isotopes, equivalent vitrinite reflectance ( Requ) and zircon (U–Th)/He ages as thermal indicators. The modeled results indicated that there are three phases of heat flow evolution in the Shuntuoguole low-uplift. (a) The gradual cooling stage during Ordovician-carboniferous. The heat flow is gradually decreasing during this period. (b) Rapidly raised stage in the early Permian period. The heat flow in Shunbei and Shuntuo area are raised rapidly during this period and the maximum is 51–65 mW/m2, but the SN3 well in Shunnan area increased slightly and the heat flow values of other single wells were still relatively stable. (c) The decline stage since Middle Permian. The tectonic activity is relatively stable and the geothermal heat flow is gradually reduced during this period, but the heat flow in Shunnan area is higher than the Shunbei and Shuntuo areas. Due to the control of thermal evolution, the dissolution of the Ordovician carbonate reservoir in Shuntuoguole area occurred earlier than the formation of large quantities of hydrocarbon materials, and the improvement of reservoir physical properties provided sufficient conditions for the late oil and gas filling, which was conducive to the preservation of large oil and gas reservoirs.

Petroleum accumulations in the tarim basin during various tectonic stages as revealed by U–Pb dating of multi-phase calcite veins in deeply buried carbonate reservoirs

Significant discoveries in deeply buried marine carbonate petroleum exploration have been made recently in the Tarim Basin, NW China. Constraining the evolutionary history of fluid activity and the process of oil charging in the deeply buried carbonate reservoirs is crucial for further hydrocarbon exploration and development. In this study, we conducted an integrated study combining petrography, geochemistry, fluid inclusion analysis, and in-situ U–Pb dating of multi-phase calcite veins collected from the Ordovician carbonate reservoirs, Akeyasu area, Tarim Basin. Four distinct phases of calcite veins were found within the Ordovician carbonate reservoirs. The U–Pb ages of these four phases of calcite veins are as follows: 459.8 ± 4.7 Ma, 418.8 ± 3.7 Ma, 329.7 ± 1.6 Ma to 310.0 ± 3.6 Ma, and 291.7 ± 6.1 Ma to 220.5 ± 7.3 Ma. These ages closely correspond to key tectonic events that influenced the study area, specifically, the Middle Caledonian, Late Caledonian, Middle Hercynian, and Late Hercynian orogenies, respectively. The first phase (C1) calcite veins are believed to have precipitated from meteoric water, closely associated with the first episode of oil charging. Phase Ⅱ (C2) calcite veins primarily formed from Early-Middle Ordovician formation water. Phase Ⅲ and phase Ⅳ (C3 and C4) calcite veins originated from Middle-Late Cambrian formation water and hydrothermal fluids, respectively. Significantly, the second and third episodes of oil charging occurred concurrently with the precipitation of C3 and C4 calcites. Seismic data analysis of Paleozoic strata in the Akeyasu area revealed four stages of fault activity, corresponding to the Early Caledonian, Middle Caledonian, Late Caledonian-Early Hercynian, and Middle-Late Hercynian orogenies. Our findings strongly suggest that the evolution of fluid activity and oil charging was primarily governed by regional orogeny. These large-scale tectonic shifts led to intensified fault activity, ultimately controlling fluid activity and oil charging dynamics. Furthermore, this intensified faulting eased the connection between deep fluid systems and the Ordovician carbonate reservoirs, serving as the primary conduits for fluid migration and oil charging within the deeply buried Ordovician carbonate reservoirs.

Controls of strike-slip faults on condensate gas accumulation and enrichment in the Ordovician carbonate reservoirs of the central Tarim Basin, NW China

Uplifiting of the central Tarim Basin has attracted great interests since the discovery of a major condensed natural gas field in Ordovician carbonate successions at a depth of > 6000 m. The formation of the large-scale hydrocarbon reservoirs in reef shoals is typically thought to be influenced by paragenetic accumulation in the karstified reservoirs. However, because of the rapid decline of natural gas/petroleum production and the complex fluid distribution in limestone reservoirs, it is difficult to clarify the controlling factors of hydrocarbon enrichment. We integrate core, seismic, well logging and production data to illustrate the relationship hydrocarbon enrichment and fault system in this paper. First, productive wells are closely associated with major strike-slip fault zones. Second, most productive blocks are distributed in a banded or block-like manner within strike-slip fault sections. Third, strike-slip fault systems do not closely control reservoir quality; however, the enrichment of natural gas is linked to the development of strike-slip fault systems. Finally, all high-productivity wells are distributed around structural highs in major strike-slip fault zones. These lines of evidence suggest that the accumulation and enrichment of hydrocarbons were closely related to strike-slip fault zones rather than kastified reservoirs. Based on this, four models for the accumulation of hydrocarbons were proposed: fault uplift, graben periphery, fault intersection and fault–karst coupling. Large-scale strike-slip fault zones, transtensional segments and local structural highs were the main factors found to be controlling hydrocarbon enrichment and high productivity. Therefore, these three conditions improved the reservoir and provided paths and places for hydrocarbon migration and accumulation. These proposed hydrocarbon accumulation models contribute to a better understanding of these fault systems and further petroleum exploration.

Critical Condition of the Depth Limit of Oil Accumulation of Carbonate Reservoirs and Its Exploration Significance in the Lower Ordovician of the Tazhong Area in the Tarim Basin

Critical Condition of the Depth Limit of Oil Accumulation of Carbonate Reservoirs and Its Exploration Significance in the Lower Ordovician of the Tazhong Area in the Tarim Basin

Hydrothermal fluid alteration of the Ordovician epigenetic karstification reservoir in the Tahe Oilfield, Tarim Basin, NW China

In recent years, the Ordovician carbonate reservoirs in the Tarim Basin have received attention due to continuous advancements in of deep strata exploration for oil and gas. The Ordovician carbonates in the Tahe Oilfield have experienced multiple phases of tectonic movement and ancient karst action to form reservoirs consisting of multi-scale spaces such as large caves, fractures, and dissolution pores. This study identifies indicators of atmospheric freshwater karst and hydrothermal karst in the Ordovician carbonate rocks of the Tahe Oilfield by comparing and analyzing lithological observation, geochemical data, fluid inclusions, logging interpretation, and seismic data. The spatial and temporal distribution of karst reservoirs are summarized, and the results show that the stage of tectonic movement and pulsating uplift of strata occurred in the early Caledonian and Hercynian orogenies, and the carbonate rocks were uplifted to the surface and large-scale atmospheric freshwater karstification.In the deeper strata, the thermal convection of fluids caused by volcanic activity accelerated the thermochemical sulfate reduction (TSR), and the generated H2S gas accompanied the upward transport of hydrothermal fluids, which further dissolved and modified the original karst system, increasing the storage space. However, away from the heat source, calcite (positive Eu anomaly, higher 87Sr/86Sr ratio, lower δ18O value,) reprecipitated due to the temperature reduction, so the role of hydrothermal activity in reservoir reconstruction is limited. The two karst action patterns indicated that epigenetic karstification is an important process for forming carbonate reservoirs in the Tahe Oilfield and the basis of hydrothermal karst reservoir formation. This study demonstrates the research and exploration value of karstification for the geo-energy field, which could benefit sustainable development in the Tarim Basin.

Segmentation and lateral growth of intracratonic strike-slip faults in the northern Tarim Basin, NW China: influences on Ordovician fault-controlled carbonate reservoirs

Intracratonic strike-slip faults have been recognized as a major factor controlling the formation of fracture-cave carbonate reservoirs in deep buried basins, yet which properties and how the strike-slip faults influence reservoir distribution and their connectivity are still ambiguous. This uncertainty significantly restricts hydrocarbon exploration and development, such as in the Fuman oilfield, northern Tarim Basin, NW China. Using a high-resolution 3D seismic reflection survey and borehole data, we investigated the geometry and kinematic evolution of the FI17 fault zone in the Fuman oilfield. This fault zone is characterized by a single fault zone, pop-up or pull-apart structures, right-stepping en echelon normal faults, and much smaller displacement (<30 m) normal fault arrays from bottom to top. The FI17 fault zone consists of four genetic segments, including the extensional strike-slip duplex, Riedel left-lateral shear, right-stepping horsetail splay, and horizontal slip segments in map view. In particular, the formation of the ∼18 km Riedel shear zone is characterized by the growth and linkage of segmented shear faults (synthetic and secondary synthetic shears). We observed that the large-scale fault-controlled fracture-cave reservoirs are distributed in positions with wider fault zones, which are characterized by overlapping of neighboring secondary shear faults. Furthermore, the reservoir width examined in this study is natural logarithmic correlated (positively) to the fault zone width. The reservoirs linked by the same shear faults show better internal connectivity. The spatial coherence between fault geometry and reservoir features indicates that segmentation and lateral growth of intracratonic strike-slip faults controls the occurrence of fracture-cave reservoirs, which may provide support for reservoir prediction in the Fuman oilfield and other deeply buried fault-controlled carbonate reservoirs in general.

Fault-karst systems in the deep Ordovician carbonate reservoirs in the Yingshan Formation of Tahe Oilfield Tarim Basin, China

Fault-karst systems in deep and ultra-deep carbonate reservoirs have a huge potential for hydrocarbon exploration. However the characteristics of reservoir space and the main controlling factors of fault-karst reservoirs are unclear due to strong heterogeneity. Therefore, the exploration and development of deep and ultra-deep carbonate reservoirs are restricted. To fill the gap, core, thin section and well log (conventional logs and image logs) data are used to analyze the lithology and main reservoir space in the deep Ordovician reservoirs in the Tahe Oilfield, Tarim Basin, China. The lithologies are dominated by intraclastic limestone, dolomitic limestone and silt to fine-sized crystalline dolostone. The reservoir space mainly includes dissolution vug, fracture, fracture-vug and cave. Among them, caves and their surrounding fractures are the main storage spaces for hydrocarbons in the Ordovician reservoir. Four fault-karst systems are divided including “V” shape, “U” shape, “layer” shape and compound type according to the morphology of the seismic section. In addition, the reservoir space of different fault-karst systems is clarified using image logs. Combined with drilling data, it is found that the reservoir scale of “U” shape fault-karst system is larger, while that of “layer” shape fault-karst system is smaller. In addition, the direction of maximum horizontal in-situ stress is NE-SW determined by induced fractures picked up by image logs. The NE orientation faults and fractures are increased and the effect of karst fluid on reservoirs is enhanced. However, the NW direction faults and fractures are restricted. Therefore, the fault-karst system is mainly observed along the NE main fault. The fluid alteration has two influences on the reservoir quality. On the one hand, the fluid alteration can dissolve the carbonate rocks and expand the reservoir scale; on the other hand, the fluid will carry mud and sand, which will reduce the reservoir space scale. The distance from the fault will affect the types of reservoir space of the fault-karst system. Away from the fault, the reservoir space will change from caves and fractures to facture-vug, and finally into small-scale dissolution vugs. Therefore, it should not only pay attention to seismic beads features but also to the size of available reservoir space on logging during drilling. This study can provide insight into the different types of fault-karst system models, and help clarify the main controlling factors of the fault-karst system scale.

In-situ stress prediction in ultra-deep carbonate reservoirs of Fuman Oilfield, Tarim Basin of China

The Fuman Oilfield in Tarim Basin has great potential for ultra-deep carbonate oil and gas resources, and is an important area for future storage and production increase. The present-day in-situ stress field is critical during the exploration and development. However, no systemic investigations have been carried out in this oilfield. Therefore, in this study, the present-day in-situ stress field in the Ordovician carbonate reservoir is predicted and analyzed based on well log calculation and geomechanical numerical modeling. The results indicate that, 1) NE-SW-trending is the dominant horizontal maximum principal stress (SHmax) orientation. The vertical principal stress is the maximum principal stress, showing the Ordovician reservoir is under a normal faulting stress regime. 2) The distribution of in-situ stresses in the Ordovician carbonate reservoir is heterogeneous, which is mainly controlled burial depth and fault/fracture development. High stress magnitudes in the Yingshan Formation are mainly in the southeastern part of Fudong area, Fuman Oilfield. The present-day horizontal differential stress mainly ranges from 27 MPa to 30 MPa in the Yingshan carbonate reservoir. iii) Natural fractures are generally stale under the present-day in-situ stress state. Fractures that parallel to the SHmax orientation with high fracture dip angle are easier to be reactivated. The results are expected to provide geomechanical references for further oil and gas development in the Fuman Oilfield of Tarim Basin.

The Strike-Slip Fault Effects on Tight Ordovician Reef-Shoal Reservoirs in the Central Tarim Basin (NW China)

The largest carbonate condensate field in China has been found in the central Tarim Basin. Ordovician carbonate reservoirs are generally attributed to reef-shoal microfacies along a platform margin. However, recent production success has been achieved along the NE-trending strike-slip fault zones that intersect at the platform margin. For this contribution, we analyzed the strike-slip fault effects on the reef-shoal reservoirs by using new geological, geophysical, and production data. Seismic data shows that some NE-trending strike-slip faults intersected the NW-trending platform margin in multiple segments. The research indicated that the development of strike-slip faults has affected prepositional landforms and the subsequent segmentation of varied microfacies along the platform margin. In addition, the strike-slip fault compartmentalized the reef-shoal reservoirs into multiple segments along the extent of the platform margin. We show that fractured reef-shoal complexes are favorable for the development of dissolution porosity along strike-slip fault damage zones. In the tight matrix reservoirs (porosity < 6%, permeability < 0.5 mD), the porosity and permeability could be increased by more than 2–5 times and to 1–2 orders of magnitude in the fault damage zone, respectively. This suggests that high production wells are correlated with “sweet spots” of fractured reservoirs along the strike-slip fault damage zones, and that the fractured reservoirs in the proximity of strike-slip fault activity might be a major target for commercial exploitation of the deep Ordovician tight carbonates.

Well trajectory optimization of ultradeep and high-pressure drilling engineering based on high in situ stress as the main control factor: A case study from the Ordovician carbonate reservoir in the Shunbei area of the Tarim Basin

With increasing oil and gas exploration and development, well trajectory optimization has gradually become the focus of the oil and gas industry. Considering the wellbore instability in the Shunbei area of the Tarim Basin, the well trajectory was scientifically optimized under the guidance of rock mechanics, drilling engineering, and mathematical methods, combined with actual geologic data, and with in situ stress as the main controlling factor. In this paper, the stress state of the wellbore is analyzed by linear elastic theory to establish the stress distribution model of the wellbore. The safety window model of wellbore stability is established using different rock failure criteria to calculate the collapse pressure and fracture pressure of the formation. Based on this, the safe mud density window is defined to achieve wellbore trajectory optimization. Finally, the influence factors of wellbore stability are discussed, and the applicability of different rock failure criteria is evaluated. The results indicate that under the normal faulting stress regime condition in the study area, the direction of horizontal minimum principal stress is the best drilling direction, where the borehole inclination angle of [Formula: see text] > 50° is the optimal well trajectory. The wellbore stabilities of high-angle deviated wells and horizontal wells are better than those of low-angle deviated wells and vertical wells. The calculation results of the Mogi-Coulomb criterion can describe the conditions of the in situ stress field more accurately. The safe windows for different well trajectories are obtained directly by the numerical method, which is very practical for optimizing well trajectories and improving wellbore stability.

Application of a model-driven simultaneous prestack inversion of rock physical properties in ultra-deep Ordovician carbonate reservoirs in the Shunbei area

Synchronous prestack inversion technology has been widely used to predict the fluid and lithological properties of reservoirs. Its prediction accuracy depends on the seismic data quality. Ultra-deep carbonate reservoirs in the Shunbei area of the Tarim Basin were used as research objects. Based on the shear wave (S-wave) prediction method, petrophysical logging analysis was carried out to select the physical property-sensitive elastic parameters. A physical property indicator with porosity as the target curve was constructed to quantitatively characterize the differences in physical properties in this area. The signal-to-noise ratio (SNR) and resolution of the dataset were improved by preprocessing the seismic and logging data. P- and S-wave impedances and densities obtained from prestack synchronous inversion, which integrate the physical property indicator model. Spatial distribution and transverse variation of the physical parameters of the Ordovician carbonate reservoirs are described. The planar distribution of reservoirs can be effectively predicted.

Identification of Karst Cavities from 2D Seismic Wave Impedance Images Based on Gradient-Boosting Decision Trees Algorithms (GBDT): Case of Ordovician Fracture-Vuggy Carbonate Reservoir, Tahe Oilfield, Tarim Basin, China

The precise characterization of geological bodies in fracture-vuggy carbonates is challenging due to their high complexity and heterogeneous distribution. This study aims to present the hybrid of Visual Geometry Group 16 (VGG-16) pre-trained by Gradient-Boosting Decision Tree (GBDT) models as a novel approach for predicting and generating karst cavities with high accuracy on various scales based on uncertainty assessment from a small dataset. Seismic wave impedance images were used as input data. Their manual interpretation was used to build GBDT classifiers for Light Gradient-Boosting Machine (LightGBM) and Unbiased Boosting with Categorical Features (CatBoost) for predicting the karst cavities and unconformities. The results show that the LightGBM was the best GBDT classifier, which performed excellently in karst cavity interpretation, giving an F1-score between 0.87 and 0.94 and a micro-G-Mean ranging from 0.92 to 0.96. Furthermore, the LightGBM performed better in cave prediction than Linear Regression (LR) and Multilayer Perceptron (MLP). The prediction of karst cavities according to the LightGBM model was performed well according to the uncertainty quantification. Therefore, the hybrid VGG16 and GBDT algorithms can be implemented as an improved approach for efficiently identifying geological features within similar reservoirs worldwide.

Constraining hydrocarbon migration and accumulation by two-dimensional numerical simulation: Ordovician carbonate reservoirs of the Daniudi Area, Ordos Basin

The various types of reservoir space in carbonate reservoirs make the process of hydrocarbon migration and accumulation very complex. How to quantitatively characterize the characteristics of hydrocarbon migration and accumulation in carbonate reservoirs is one of the key problems for carbonate hydrocarbons exploration and exploitation. Taking the Ordovician in the Daniudi area of Ordos Basin as an example, three types of source rock-reservoir-cap assemblages, namely upper-generation assemblage, lateral variable assemblage, and self-generation-reservoir-cap assemblage, were determined by characterizing the forming elements of carbonate reservoirs. Petromod software is used to build a two-dimensional profile model, and vitrinite reflectance of source rock and porosity of the reservoir were used as correction parameters, and the hydrocarbon migration and accumulation process was simulated by the invasion percolation algorithm. It is confirmed that the Ordovician Majiagou formation in the study area has three periods of hydrocarbon charging and three periods of hydrocarbon adjustment. The hydrocarbon migration pathways in carbonate rocks include lateral contact in the pinch-out area, source rock-reservoir contact in the trough area, fault-connected, and source rock-reservoir superposition. The source rock-reservoir-cap assemblages of upper-generation assemblage and lateral variable assemblage correspond to the lateral contact in the pinch-out area, source rock-reservoir contact in the trough area, fault-connected, while self-generation-reservoir-cap assemblage corresponds to the fault-connected, and source rock-reservoir superposition. The high part of the structure and fault zone are the strongest structural deformation parts, whose fractures are relatively developed, controlling the location of hydrocarbon accumulation, meanwhile, the reservoir quality directly controls the degree of hydrocarbons saturation. The research results are beneficial to establish a more accurate quantitative evaluation method of hydrocarbon migration and accumulation in carbonate rocks and to understanding the controlling factors of hydrocarbon migration and accumulation in carbonate rocks.

Characteristics and Seismic Identification Mode of Ultra-Deep Carbonate Fault-Controlled Reservoir in Northwest China

Ordovician carbonate reservoirs in the Shunbei area of the Tarim Basin are characterized by large burial depth, small vertical fault displacement of the strike-slip fault zone, small karst development scale, strong reservoir heterogeneity, complex and diverse seismic reflection types, which lead to unclear seismic response characteristics of fault zone and reservoir, and further restrict the fine description of ultra-deep (>7300 m) fault controlled reservoirs. On the basis of high-precision 3D seismic data analysis, combined with drilling, logging, and comprehensive geological interpretation data in the Shunbei area, seismic geological models of different types of reservoirs and fault zones are established. The changes in seismic response characteristics of fault zones and reservoirs are simulated and analyzed by changing reservoir size, scale, fault and fracture cavity combination modes, as well as other factors. On this basis, two seismic identification models of fault-controlled reservoirs in the Shunbei area are summarized and established. The seismic identification mode of seismic reflection marker wave of fault-controlled reservoir is mainly “fault seismic response characteristics & weak reflection” and red trough anomaly under T74. The internal seismic identification modes are mainly “fault seismic response characteristics & random reflection”, “fault seismic response characteristics & beading”, “fault seismic response characteristics & linear weak reflection”, and “fault seismic response characteristics & random reflection”. Among them, except for “fault seismic response characteristics & random reflection”, the coincidence rate of reservoir prediction of other three types of seismic response characteristics is more than 70%. The coincidence rate of the other three types of seismic response characteristics of a reservoir is more than 70%. The research results provide a reference for the prediction and description of ultra-deep carbonate fault-controlled reservoirs.

Characteristics and application of present in situ stress field of a strike-slip fault: a 3D finite-element simulation study

AbstractPrevious hydrocarbon explorations in the middle of the Tarim Basin indicate that strike-slip faults play an important role in the development of Ordovician carbonate reservoirs and hydrocarbon accumulation. The SB5 fault in the Tarim Basin was the target of this investigation. An evaluation of the stress in situ was carried out and provided boundary conditions to build a 3D geomechanical model. The distribution and application of present in situ stress in the strike-slip fault were studied. The results show good agreement between the absolute measured stress in situ and the modelled stresses, revealing a different stress regime along the strike-slip fault. The uplift segment belongs to a strike-slip stress state, and other areas belong to a normal fault stress state. The strike-slip fault has a significant influence on the present in situ stress distribution. The direction of the maximum horizontal stress deflects near the fault and tends to be parallel to the fault strike. This work introduces a comprehensive evaluation of the present in situ stress of the fractured carbonate reservoirs controlled by the strike-slip fault system. The present in situ stress direction can clarify the propagation direction of hydraulic fracturing and serve to evaluate the effectiveness of natural fractures.

The Advancement and Challenges of Seismic Techniques for Ultra-Deep Carbonate Reservoir Exploitation in the Tarim Basin of Northwestern China

The largest ultra-deep (>6000 m) strike-slip fault-controlled oilfield in the world is found in the Tarim Basin of Northwestern China. The localized fractured reservoirs are the major production targets along the strike-slip fault zones. Different from its use in the primary porous-type reservoirs, however, the conventional technology is not favorable for use in oil/gas development in Ordovician carbonate reservoirs. For this reason, high-density seismic acquisition and high-resolution seismic processing were carried out to provide high-precision data for fault and fractured reservoir identification. In addition, the multi-filtering process and the maximum likelihood method are typically used to identify small faults and fault segments along a strike-slip fault zone. Further, seismic facies-constrained inversion and amplitude attributes are favorable for large fracture-cave reservoir description. With the advancements in seismic technology, the high and stable production well ratio has been doubled in the “sweet spots” of fractured reservoir optimization, and the first ultra-deep strike-slip fault-controlled oilfield with an annual oil production of over 1 million tons has been realized, achieving economic development in the ultra-deep fractured reservoirs. However, unstable production and high rates of production decline are still significant challenges in the economic exploitation of the ultra-deep fractured reservoirs. Seismic technology requires further improvement for the description of small fractured reservoirs and matrix reservoirs, as well as reservoir connectivity prediction and hydrocarbon detection in the deep subsurface.

“Second quantitative characterization” and its application in fractured-vuggy carbonate reservoirs

The Ordovician fracture-vug carbonate reservoirs of Tarim Basin, are featured by developed vugs, caves and fractures. The strong heterogeneity results in huge uncertainty when these reservoirs are quantitatively characterized using merely static seismic data. The effective quantitative characterization of the reservoirs has been an urgent problem to be solved. This study creatively proposes the “second quantitative characterization” technique with the combination of dynamic and static data based on the primary static quantitative characterization and fully considering lots of key influence factors when conducting characterization. In this technique, dynamic analysis methods such as well testing, production rate transient analysis, dynamic reserve evaluation and dynamic connectivity evaluation are used to get understandings on this kind of reservoir. These understandings are used as statistical parameters to constrain the inversion of seismic wave impedance to improve the relationship between wave impedance and porosity and determine the fracture-vug morphology, calculate dynamic reserves, and then a more accurate fracture-vugmodel can be selected and used to calculate the oil-water contact inversely based on the results of “second quantitative characterization”. This method can lower the uncertainties in the primary quantitative characterization of fracture-vug reservoirs, enhance the accuracy of characterization results significantly, and has achieved good application results in the fracture-vug carbonate reservoirs of Tarim Basin.

Characteristics, Diagenesis, Controlling Factors and Formation Mechanism of Deep-Burial Ordovician Carbonate Reservoirs in the Yangshuiwu Area, Jizhong Depression, Bohai Bay Basin

Characteristics, Diagenesis, Controlling Factors and Formation Mechanism of Deep-Burial Ordovician Carbonate Reservoirs in the Yangshuiwu Area, Jizhong Depression, Bohai Bay Basin