With the development of oil and gas exploration to deep layers, ultra-deep layers below 7 000 m have become the focus of exploration. The formation, preservation and prediction of ultra-deep ancient carbonate reservoirs are key problems. Based on the analysis of the drilling data below 7 000 m of Ordovician in Tarim Basin, China, this paper characterizes the development characteristics and formation and evolution process of ultra-deep carbonate reservoirs. The formation of ultra-deep carbonate reservoirs is controlled by high-energy depositional bodies, penecontemporaneous exposed karstification and the reconstruct of faults. Through seismic attribute extraction, beads were found and a method of drilling along the fault zone along the karst layer was proposed. The success rate of drilling was increased to 95%. The quantitative sculpture and characterization technology of fracture-cavity has been established, and hydrocarbon accumulation regularity and well point optimization method for fracture-cavity reservoirs have been formed. Mechanical models of vugs in fault and fracture-cavity carbonate reservoirs are established, and numerical simulation and theoretical analysis are carried out. The results show that the distance from fault plane increases, and the fracture development ability decreases. Over 8 500 m is the dominant distribution area of carbonate reservoir, 8 500-9 200 m is the effective distribution area, 9 200-9 500 m is the general distribution area, below 11 000 m, large vugs are gradually disappearing. Parabolic pressure arches are formed at the top of the paleocave in the ultra-deep carbonate reservoir in the platform basin area to prevent the cave from collapsing completely. Take the parabolic cave with the height-span ratio of 1 and arrow height 8 m as an example, its fully closed depth is over 50 000 m. Therefore, the lower limit of exploration depth for carbonate cave reservoirs far exceeds the current drillable depth.
There are three types of dolomites in Penglaiba Formation of Yonganba outcrop in the Bachu area of the Tarim Basin. Based on the analysis of the sedimentary context of this area, and the characteristics of dolomite and XRD results, it could be suggested that three types of dolomites represent different sedimentary environments and process. The average ordering degree of powder crystal dolomite is 0.75 and their unit cell parameters deviate from that of ideal dolomite, which means they are the products of lime-muds by both rapid dolomitization in penecontemporaneous stage and recrystallization in shallow burial stage. The average ordering degree of the fine-middle crystal dolomite is 0.79 and their unit cell parameters are close to ideal dolomite, indicating the effects from porous grain limestones with both dolomitization in penecontemporaneous stage and adjustment to burial dolomites during the burial stage. The average ordering degree of coarse crystal dolomite is 0.90 whose unit cell parameters are close to the ideal dolomite, revealing their original rocks are granular limestones influenced by the persistent and sufficient dolomization fluids, with the slower crystallization speed and longer dolomization period. Preservation of pores and formation of effective reservoirs in the fine-middle crystal dolomites were more common than in other two types, because of the disequilibrium dolomitization.
Since the breakthrough made in Well GC6, the Ordovician Yingshan Formation in Tarim Basin has become a major exploration formation, with the third member of Yingshan Formation (O1y3) to be one of the producing zones. It is proved by wells that siliceous content is commonly high in Ordovician of Gucheng area. Based on 3D seismic and well data, basic theory and methods of seismic sedimentology were introduced to calculate distribution of siliceous quantitatively and to characterize features of reservoir and faults in the target layer by such techniques as -90° phase rotation, frequency decomposition, attribute generation, RGB color blending, principal component analysis, regression fit, et al. Relationship between siliceous content and reservoir and faults was discussed. In order to realize the purpose of quantitatively calculation of siliceous rock and qualitatively characterizing of reservoir and faults, the above methods were combined into an eight-step working flow. Results show that the siliceous is very popular in the O1y3 (mainly in the lower of O1y3, i.e. O1y3L). According to statistics from well data, thickness of high-siliceous rock varies from 7.13 m to 89.50 m, averaging 55.02 m. The siliceous rocks are mainly distributed in the east of the study area, which can be more than one hundred meters thick according to the seismic data. Faults, especially long-period active faults are the main controlling factor of the distribution of siliceous. Thickness of high-siliceous rocks is obviously larger than that in other areas. Relationship between siliceous and reservoir is much more complicated. It seems that, effective reservoir is usually located in areas where the high-siliceous rock become thinner.
The northern tectonic belt of Kuqa Depression has an enormous potential for hydrocarbon exploration, but the reservoir physical property of Middle and Lower Jurassic has significant horizontal difference and the controlling factors are not clear, which constrain the prediction of high quality reservoir. Based on outcrops, cores, thin sections and image logging data, petrology characteristics, reservoir space types, physical property of Lower Jurassic Ahe Formation in the northern tectonic belt of Kuqa Depression were described, and the characteristics and formation sequence were analyzed. Then, based on above, the controlling effects of sedimentation, diagenesis and paleo-tectonic stress on reservoir were discussed, and the dominant factor of the horizontal difference of reservoir was confirmed. The results show that Ahe Formation is mainly composed of grey-white medium sandstone, coarse sandstone and conglomerate lamina, and the sandstones are primarily feldspar lithic sandstone and lithic sandstone. The reservoir spaces are mainly intragranular dissolved pore, micro fracture and original intergranular pore in outcrops, while mainly micro pore and intragranular dissolved pore underground. The reservoir physical property has significant lateral differences. The Ahe Formation primarily develops vertical and high angle shearing fractures, and the fractures have lower filling extent. Micro fractures are chiefly grain-edge micro fractures in outcrops while mainly intragranular micro fractures underground. There are three periods of fractures in the study area, and the fractures formed at the Late Himalayan have the best effectiveness. The deposition dominates reservoir physical property of Ahe Formation, and the compaction is the primary effect of porosity loss while the dissolution is the chief effect of porosity increase. However, they are not the dominant factor for reservoir horizontal differences. The paleo tectonic stress has a double effect on reservoir. One effect is reducing porosity and permeability, and the other effect is forming fractures and increasing permeability, which dominate the horizontal reservoir differences of Ahe Formation.
The deep layer carbonate rock of Ordovician (the 3rd-4th members of Yingshan Formation and Penglaiba Formation) is one of the important exploration fields in Tarim Basin. In 2018, Well G70 in the north slope of Tazhong obtained high-yield industrial gas flow in the test of the fourth member of Yingshan Formation of Lower Ordovician, and the deep layer of Ordovician in the northern slope of Tazhong Uplift showed broad exploration potential. The key problem restricting the selection and target optimization of the deep exploration of Ordovician System is the poor knowledge about the reservoir characteristics, forming mechanism and reservoir distribution. Based on the analysis of single well logging, well logging, well testing and well testing dynamic data and comprehensive utilization of seismic data, the main controlling factors and high-quality reservoir distribution of the 3rd and 4th members of Yingshan Formation of Tazhong Uplift are developed in this paper. It is found that there are various types of rocks in the 3rd and 4th members of Yingshan Formation on the northern slope of the Tazhong Uplift, including limestone, dolomite transition rock and dolostone. The reservoir space type is cave, vug-pore and fracture. Cave type and fracture type reservoirs are the main reservoir types in the study area. The dissolution fabric selectivity, karst landform and feather strike slip fault control the high-quality reservoir components. In addition, the reservoir control model of interlayer karst superimposition strike slip fault is established. According to the seismic reflection characteristics of high-quality reservoir, it is pointed out that the most favorable reservoir development area of the third and fourth members of Yingshan Formation in the study area is mainly concentrated in the development area of pinnate fracture zone of karst slope superimposed strike slip fault in the north of Tazhong No.10 belt, which is the next exploration preferred area.
The Lunnan low uplift of the Tabei Uplift is a hotspot for oil and gas exploration in the Tarim Basin, in which a series of fractured-vuggy oil and gas reservoirs controlled by faults are found. At present, the faults in the study area are mostly studied in a certain block, which needs to be systematically hackled in the whole study area. Based on a large number of three-dimensional seismic data, the method of fault structure analysis is adopted to analyze the types, structural features, and formation and evolution processes of faults in the study area. The results show that: There are three types of faults, such as reverse faults, strike-slip faults, and transtensional faults. The reverse faults are distributed in the northwest of the study area and central Lunnan areas. There are both detachment thrust faults and basement-involved thrust faults. Strike-slip fault is the main fault type in the study area. Three kinds of structural styles are developed on the profile, which are high-steep style, positive flower, and semi-flower. Four kinds of structural styles are developed on the plane, which are linear extension, chess board format, braided structure, and horsetail structure. The strike-slip fault has obvious segmentation along the strike. The transtensional faults are mainly small, which are distributed in the Mesozoic and Cenozoic. The transtensional faults are stepped, grabbed or negative flower on the profile. On the surface, most of them are arranged in the form of en echelon along the strike-slip faults or the reverse faults. Three methods are used to reveal the evolutionary history of strike-slip faults, reverse faults, and transtensional faults respectively, which are the differences of upper and lower fault structure styles, the age of strata deformed by faults and growth index. The faults have undergone 4 stages, which are the middle Caledonian, the early Hercynian, the late Hercynian-Indochina, and the Yanshan-early Himalayan. The fault evolution has certain inheritance.
Faults are important oil and gas reservoir spaces and seepage channels, and faults are hotspots in oil and gas geological research. A series of NE-trending strike-slip faults develop in the Tarim Basin. The structural evolution and formation mechanism of these faults need to be studied further. Based on the 3D seismic data in the Xiaotangnan area of the northern slope of Tazhong, the strike-slip fault structure characteristics, formation evolution and mechanism analysis were accomplished. The results are that: (1) Two sets of fault systems, namely deep strike-slip faults and shallow tensile torsional faults, are developed in the Xiaotangnan area. The deep strike-slip fault profile is a high-steep upright or regular flower-like structure, and on the plane, most of them show linear extensions and some branch faults develop at the intersection and the bend. The shallow transtensional faults have a negative flower-like structure on the cross section, and they are divided into three groups of faults: NW20°, NE10°, and NE30°. (2)The faults in the study area experienced two phases of activities: Late Ordovician strike-slip faults, and Early Carboniferous transtensional faults. The Late Ordovician strike-slip faults are transpressional strike-slip faults which are generated by horizontal displacement along the NE-preserved basement weak belts with the strong compression of the southwestern margin of the basin. The Early Carboniferous transtensional faults are syn-shear faults, tensile t-fractures and branch faults caused by block rotation which are generated by reactivation of deep strike-slip faults that are closely related to the oblique compression from the Algin tectonic domain in the southeastern margin of the basin. The result has certain guiding significance for the exploration of Ordovician fracture?cavity oil and gas reservoirs in this area.
Cambrian source rocks in Tarim Basin have been the focus of many scholars' research in recent years. There are still some divergences on their distribution, which become an important factor restricting Cambrian exploration and zoning optimization. Based on the knowledge of outcrop and drilling data in the basin, the sedimentary environment of Cambrian source rocks is analyzed in this paper. According to the sedimentary environment and development time, the Cambrian source rocks are divided into mild-slope style rocks and steep-slope style rocks in East Tarim. The mild-slope source rocks, deposited in the same period of the Yuertus Formation, is mainly controlled by the pre-Cambrian palaeogeomorphology. The thickness center is located in the northwest side of the Manjiar Depression, and the thickness of the source rocks is about 15-40 m. The steep-slope source rocks, developed in the sedimentary environment of the platform margin slope-basin, is located in the Lunnan-Gucheng arc belt in Manxi area, with a maximum thickness of about 150 m. Based on the distribution characteristics of two types of source rocks in East Tarim area, dolomite of Lunnan area and the Middle-Upper Cambrian platform margin belt in the north of Gucheng low uplift are selected as favorable exploration zone in East Tarim Basin.
Based on the field outcrop and drilling data, combined with the stratigraphic distribution and lithofacies associations of Sinian-Cambrian, it is proposed that “Chengkou-Badong-Wufeng” intracratonic rift develops in western Hubei-eastern Chongqing. The rift shows an “hourglass” shape, spreading from south to north with a width of 60-280 km from east to west and a length of about 400 km from north to south. The rift was formed in Doushantuo Period of Sinian, and developed in succession in Dengying Period. In the early stage of Early Cambrian, it was the peak period of rifting development, and it declined in the middle and late stage of Early Cambrian, and died out in the Middle Cambrian. Under the control of the evolution of the rift, there are thick and high quality source rocks in the Doushantuo Formation and Qiongzhusi Formation. The Dengying Formation and Longwangmiao Formation have high quality mound-shoal facies and grain shoal facies dolomite reservoirs on both sides of the rift, among which the Dengying Formation has a reservoir thickness of 55-100 m and the Longwangmiao Formation has a reservoir thickness of 22-57 m. It is pointed out that Wuxi-Fengjie area is located in the superimposed development area of mounds and shoals reservoir of Dengying and Longwangmiao formations, adjacent to hydrocarbon generation center, which is easy to form two effective accumulation combination, one is the side product and side reservoir migrates toward the sides, the other is lower product and upper reservoir migrates vertically. And it should be the target area for further exploration in East Sichuan.
O1m54-1a is a new formation of Ordovician natural gas exploration in Ordos Basin, which is expected to be another area with large-scale increase of reserves and production after O1m51+2 weathering crust gas reservoir. Based on drilling core, thin section, cathodoluminescence, physical property data, and geochemical characteristics, genesis and distribution of the reservoirs of the O1m54-1a in the central and eastern parts of the Ordos Basin are studied. The results showed: (1)The lithology of the reservoir of the O1m54-1a is dolomudstone with nodular anhydrite. The storage space is mainly the dissolution of anhydrite moldic pores, containing a small amount of anhydrite crystal moldic pores and micro-cracks, and the average porosity is 5.54%; the average permeability is 1.53×10-3 μm2. (2)The reservoir pore experienced anhydrite precipitation, and atmospheric fresh water dissolution, and mesocrystalline calcite filling, and quartz filling, and medium coarse crystal dolomite filling, and megacryst calcite, and rupture; (3)Differential dissolution and filling controlled by paleogeomorphology control the plane distribution of reservoir. The comprehensive analysis shows that the congruent area between high part of sedimentary paleogeomorphology and karst slope locating at the west of Yulin-Jingbian-Yan'an and the Hongdunjie-Qingyangcha area has strong dissolution and low filling degree, which is the distribution area of high-quality reservoir, providing theoretical basis for the next exploration and deployment.
Lower Cambrian Xiaoerblak Formation is an important exploration target of Cambrian pre-salt in Tarim Basin, but the limited knowledge of sedimentary facies and reservoir genesis led to the restriction of exploration. This study took Xiaoerblak outcrop of Keping area as an example. Based on detailed description, 110 thin sections identification and multi-parameters geochemical (order degree, trace elements, rare earth elements, δ13C, δ18O, 87Sr/86Sr, U-Pb dating and cluster isotopes) data analysis, it was found that Xiaoerblak Formation can be divided into three members, and microbial dolomite is the main lithology. The characteristics of lithofacies assemblage form bottom to up indicate that it can be described as a complete three-order sequence. The sedimentary environment of Early Cambrian Xiaoerblak Period in Keping area characterized by warm-dry climate, normal-higher sea salinity, gradually increase oxygen content and upward shallower water. The sedimentary sequence was microbial layer-microbial mound/shoal-tidal flat in carbonated ramp background. Dolomitization took place in penecontemporaneous-early diagenetic stage and the main dolomitization fluid was high-saline seawater. Primary microbial framework pores and vugs which formed by the dissolution atmospheric water are the main reservoir spaces. Reservoir was mainly controlled by sedimentary microfacies, high frequency sequence interface and early dolomitization. These research results are of great significance for sedimentary facies mapping and reservoir prediction in Cambrian pre-salt field.
The 3rd Member of Yingshan Formation is a main reservoir section in Gucheng area of Tarim Basin, but the development of siliceous rock destroyed the reservoir to a certain extent. The siliceous rocks in the study area are mainly residual structural rocks. The results of geochemical analysis show that the siliceous rocks are located in the hydrothermal sedimentary area in the Al-Fe-Mn diagram, and the values of Al/(Fe+Mn+Al), (Fe+Mn)/Ti, Y/Ho-Th/U, ΣREE, LREE/HREE, δCe and δEu are consistent with the geochemical indexes of hydrothermal sedimentary siliceous rocks. The Si isotopes, O isotopes and their correlation are consistent with the geochemical indexes of metasomatic siliceous rocks and diagenetic quartz. The above characteristics indicate that the siliceous rocks of the 3rd Member of Yingshan Formation in the study area are formed by hydrothermal metasomatism after diagenesis. Combined with the regional geological data, the NE trending faults that break through the basement developed in Early Caledonian Period, providing a migration channel for the hydrothermal fluid. At the end of Permian, large-scale volcanic activities took place in Tarim Basin, which provided hydrothermal and siliceous sources for silicification of carbonate rocks. The siliceous hydrothermal fluid entered the 3rd Member of Yingshan Formation along NE-trending faults and its associated fractures and reacted with carbonate rocks. The siliceous hydrothermal fluid replaced carbonate sediments and precipitated to form siliceous rocks.
Hydrocarbon source rocks are the material basis for oil and gas exploration, and their quality and quantity determine the oil and gas resource potential. Sedimentary environment reflects comprehensive information about primary productivity, nutrient supply, aqueous redox conditions, and paleoclimate, thus exerts a dominant control on the formation and distribution of high-quality source rocks. However, the burial depth and thermal maturation of these source rocks significantly increase as hydrocarbon exploration continuously expands into deep and ultra-deep strata, making many conventional geochemical proxies such as biomarkers and kerogen maceral analysis lose indicative significance for sedimentary environment. Stable nitrogen isotopes which record the original signals of marine nitrogen cycles are reliable indicators that can be used to reconstruct the paleomarine environment, explore links between environment and biological evolutions through the geological history, and indicate climate changes. In addition, this method also shows great application potential in oil and gas fields such as sedimentary environment reconstruction of ancient hydrocarbon source rocks and oil-source correlations. On basis of full investigation, this study systematically summarizes marine nitrogen cycle processes, nitrogen isotope fractionation mechanism, analyzes nitrogen isotope distribution and its controlling factors.
In order to better characterize the geological and seismic characteristics of the carbonate mound-shoal complexes of the Xiaoerbulake Formation in the Tarim Basin and provide a basis for prediction, a large-scale outcrops area with 28 km long in the Keping area in the northwestern Tarim Basin is taken as the research object. Based on seven outcrops description, more than 1 000 thin sections identification, and 56 petrophysical acquisition and analysis, internal structure and lateral distribution features of the mound-shoal complexes were systematically studied, and the outcrop geological model at the seismic scale was established. The seismic forward modeling study was conducted based on the outcrop geological model. The results show that the mound-shoal complexes are developed in the sedimentary system of inner-ramp face in ramp background of Xiaoerbulake Period. The mounds are mainly composed of algae mounds with bonding structure and foamy-stromatolite mounds, the shoals are mainly composed of algal sandy shoal. The results of seismic forward modeling show that the high-frequency components of seismic data can characterize the internal structure and external shape of mound-shoal complexes. The mounds often show the features of high frequency, high amplitude, more continuous reflection, mound shape and filling style. While the shoals often show the features of mid-high frequency, mid-high amplitude and continuous mat reflection, internal skew overlap. The seismic reflection characteristics of the mound-shoal complexes revealed by the outcrop geological model-seismic forward modeling have achieved good results in the seismic characterization of mound-shoal complexes of the Xiaoerbulake Formation in the 3D area of Tazhong.
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