Searching for oil and gas in ultra-deep carbonate sequences in marine basins is becoming one of the major trends of future petroleum exploration in China. In recent years, major oil and gas discoveries have been made in the exploration of deep and ultra-deep sequences in petroliferous basins such as Tarim, Sichuan and Erdos. Based on analysis of the basic characteristics of the typical ultra-deep carbonate reservoirs in Tahe oilfield and the deep northern slope in Tazhong of Tarim Basin and Yuanba gas field of Sichuan Basin, it is concluded that hydrocarbon accumulation in ultra-deep marine carbonate is mainly controlled by 4 basic factors: (1) High quality source rocks. In low geothermal settings, source rocks of various biological combinations and lithologic types generated abundant hydrocarbons via multiple mechanisms such as kerogen, ancient oil reservoirs and dispersed dissoluble organic matter; (2) High quality reservoirs. The development and distribution of high quality reservoirs are jointly controlled by structure, sequence, lighology, fluid and their timing, among which faulting, dolomitization and thermal fluid activity are especially crucial to the formation of high quality ultra-deep reservoirs; (3) Multiple sealing mechanisms. Regional caprock, local caprock and direct caprock of various lithologies provide favorable conditions for the sealing and preservation of hydrocarbons; (4) Favorable hydrocarbon migration pathways. Effective combinations of unconformities, faults and carrier beds with various types of traps on paleo-highs and paleo-slopes determine the pattern and efficiency of hydrocarbon migration and accumulation. Ultra-deep marine carbonates in China have huge oil and gas resource potential, predominantly gas potential. In Tarim Basin, the exploration domains of the ultra-deep carbonates mainly distribute in the Cambrian and Ordovician in the pitching ends, slope zones and sags outside the structural highs of large uplifts; in Sichuan Basin, they mainly occur in the lower marine sequence assemblage outside central Sichuan and in the upper marine sequence assemblages in foreland areas of western and northeastern Sichuan; in Erdos Basin, they mainly include the Ordovician of Tianhuan syncline and thrust front.

In this paper, a comprehensive geology-geophysics-geochemistry research method was used to reveal the theories of oil and gas formation and distribution in deep marine carbonates in Sichuan superimposed basin under the effect of (ancient) deep burial depth and longterm (multi-phase) structural difference with multi-stage structural evolution and multi-phase reservoir formation process in superimposed basins as the main research line. And two theories were proposed on the formation of marine oil and gas in deep zones in the Sichuan superimposed basin and they are the four-center coupling reservoir formation theory and the three-level three-element joint control theory. It is indicated that the formation of gas reservoirs is controlled by the coupling relationship of four hydrocarbon centers (hydrocarbon generating center, gas generating center, gas storage center and gas retention center) under the effect of multi-phase tectonism, and the ultimate distribution of oil and gas is dominated by the spatial distribution relationship of three centers (gas generating center, gas storage center and gas retention center). Oil and gas distribution is controlled in terms of basic conditions and exploration prospects by three basin-scale (the first level) factors (developed source rocks, highly matured organic and high-quality sealing conditions). The favorable hydrocarbon distribution zones are jointly controlled by three play-scale (the second level) factors (intracratonic sags, paleo-uplifts and basin-orogenic belt systems). And the distribution of large-scale gas fields is dominated by three trap-scale or reservoir-scale (the third level) factors (trap closure, seal strength and hydrocarbon infilling richness). Both four-center coupling reservoir formation theory and three-level three-element joint control theory are significant for the exploration and development of deep gas in Sichuan superimposed basin and other superimposed basins in China.

Porosity and permeability heterogeneities of reservoirs are common geological phenomena that influence significantly fluid flowing. They are very important not only for oil and gas production but also for oil and gas migration and accumulation researches. In this paper, a kind of special structural heterogeneity was discussed. Owing to the heterogeneities of sedimentary structures in reservoirs, a certain 3D spatial structure is formed by various tight intercalations. Due to the existence of such 3D structures, the flow of fluids in reservoir is impeded, diagenetical heterogeneity is induced during the burial history, and strong heterogeneities are generated in the process of hydrocarbon accumulation and porosity reduction. It is shown from simulation results that this kind of structural heterogeneity can result in strong heterogeneity of migration pathways and accumulation locations. Based on core observation and hydrocarbon reservoir dissection analysis, diagenesis and reservoir formation took place alternatively when Jurassic oil-bearing reservoirs in the central Junggar Basin were buried to the deep zones in the basin. Due to the existence of structural heterogeneity, the high-quality reservoirs can be preserved in the deep zones and it is favorable for the migration and accumulation of oil and gas at the later stage.

Kuqa foreland basin is one of the hot spots for ultra-deep hydrocarbon exploration in China and even around the world, where a number of large-scale gas fields of high and steady production have been discovered in the subsalt clastic fromation 6500m to 8000m deep. The progress in geological understanding is crucial in finding out the direction for the exploration of ultra-deep reservoirs. ① As a result of the depressuring effect of roof structure, there are large-scale effective reservoirs with a porosity of 3% to 9% existing in the subsalt . “Stress neutral plane” effect formed in the subsalt fault anticlines under the strong extrusion stress, controls the vertical zonation of the reservoirs, making high productivity reservoirs occur on the top. ② A series of thrust belts are formed in the subsalt under the intense squeezing action of the foreland thrust belt, where abundant fault anticline traps are in rows and belts. ③ The Paleogene gypsum-salt caprocks in plastic flow state covering on the fault anticline traps, providing effective sealing. The ultra-deep reservoirs in Kuqa foreland basin have favorable petroleum geological conditions and bright exploration prospect, and Qiulitage structural belt, in the southern Kuqa foreland basin, is expected to be a relay exploration domain of ultra-deep hydrocarbon.

By analyzing formation and evolution processes of major gas reservoirs in marginal reefs of Sinian Dengying Formation in Gaoshiti Area of the Sichuan Basin, in Permian —Triassic marginal reefs on both sides of the Kaijiang-Liangping Marine Trench and in Ordovician marginal reefsin Tazhong-I slope-break belt of the Tarim Basin, reservoir formation modes and major controlling factors for large-scale gas reservoirs in marginal reefsof carbonate tablelandhave been studied to establish reliable foundation for exploration works. Within the Sichuan Basin and the Tarim Basin, structural patterns generated by joint development of faults and uplifts in carbonate tableland and their evolution controlled not only spatial distribution of hydrocarbon source rocks, reefs and high-quality reservoir formations in karst formations, but also spatial combination of generation, preservation and capping formations. Eventually, these structural patterns may determine the formation of large-scale gas fields in marginal reefs. In other words, joint development of faults and uplifts may control sources, phases, preservation and reserves in relevant reservoirs. Aulacogen may control spatial distribution of high-quality hydrocarbon source rocks to form hydrocarbongeneration center; Slope break belts around joint sections of faults and uplifts may control development and spatial distribution of reefs. Consequently, favorable reef belts developed along such slope-break belts may be formed; Subsidence and uplifting of faults and uplifts may generatekarstification of various types in different phases. Consequently, high-quality karst reservoir formations of fracture-cavity type can be formed on such marginal reefs. Generally speaking, formation of large-scale gas reservoirs in marginal reefs of carbonate tableland can be divided into 6 stages: formation of structural patterns with joint development of faults and uplifts, formation of reef structures, formation of capping formations, formation of oil reservoirs in marginal reefsand formation of gas reservoirs in such marginal reefs. To explore large-scale gas reservoirs in marginal reefsof carbonate tablelandprinciples with “Identification of trench (source), selection of uplifts and exploration of reefs” shall be followed.

In Junngar Basin, a series of large anticliens and faulted anticlinal structures are developed in Carboniferous and Lower Permian in Mahu sag and its periphery. They are distributed at the center of of high-quality hydrocarbon kitchen of the Lower Permian Fengcheng Formation and its perihery in Mahu sag, where the conditions of resrvoir formation superior, so they are important exploration areas of deep oil and gas in Junggar Basin. Due to the deeper burial depth of these structures, however, the exploration and development potential in these areas is mainly affected by the following two aspects. The first is whether effetive reservoirs are developed in a large scale in the target layers. And the second is whether it is gas-phase hydrocarbon. It is indicated from comprehensive analysis of aeromagnetic anomaly processing and interpretation, seismic horizon velocity analysis, source rock evolution and natural gas genetic classification that volcanic weathering crust reservoirs are developed at Mabei anticline, Well Da 1 anticline and central-northern Mahu anticline. Based on evolution stages of Fengcheng Formation source rocks, spatial locations of anticlinal structures and oil cracking depth, it is predicted that the hydrocarbon phase is kerogencracking gas in Mabei and Mahu anticlines, oil-gas paragenesis in Well Da 1 anticline and dominantly gas in Manan anticline.

High risks and high costs of deep-target drilling make the prediction role of deep seismic prospecting very important. In view of issues such as long travel distance and complicated travel routes of seismic waves, weak energy of effective signals and severe loss of high frequency reflection signals, complex geological conditions and high requirements on acquisition accuracy in deep formation seismic survey, we have come up with the acquisition technical idea of “facing target, oriented to processing, and making use of low frequency data”, and divided the deep targets into 2 groups, complicated structure targets (CST) and complicated reservoir targets (CRT) to investigate accordingly. The first and foremost mission in studying CST is raising the signal to noise ratio of the seismic data. Wide-line large-array 2D acquisition has solved the problem firstly, and contributed to the continuous breakthroughs of deep sub-salt structure exploration in Kuqa thrust-belt of Tarim basin. The key point in studying CRT is enhancing overall seismic data precision, "WBH" (wide azimuth, broad band and high density) 3D acquisition is the inevitable choice for this issue, which has facilitated the rapid evaluation and productivity construction of the deep karst reservoirs in Halahatang area, Tabei uplift. In summary of the requirements in both deep geological research and high-precision processing, it is concluded that "making full use of low frequency data" will become the direction of deep formation seismic exploration in the near future.

About 90% of oil and gas resources in Kuqa foreland basin are in deep and ultra-deep formations, but in this area with complex geological conditions, oil and gas reservoirs are big in burial depth (5000-8000m), high in temperature (100~178℃ ) , and pressure (100 ～ 140MPa). The upper formation with high-steep structures and thick gravel layers, pose great challenge to anti-deviation and ROP (Rate of Penetration) enhancement; the middle and lower sections consisting of salt and gypsum layers, ultra-high-pressure saline water layer (2.4 - 2.6g/cm³), and low pressure lost circulation formation, have variable pressure systems and thus high risks; abundant fractures, strong tectonic force, strong water sensitivity and high abrasion of the reservoir bring about a series of problems such as drill pipe sticking and low ROP etc. In order to solve the problem of increasing ROP in high -steep structural, Tarim Oilfield has introduced and improved the vertical drilling system and worked out the principle of bit selection, forming a corollary technique series for increasing ROP in high-steep structures. In view of the problem of salt and gypsum formation, Tarim Oilfield has introduced and improved the imported oil based drilling fluid, forming ultra-deep salt & gypsum formations drilling technology centering on oil base drilling fluid; aiming at the difficulty in enhancing ROP in reservoir formation, ROP enhancement tools have been introduced from abroad. With the combination of the above key technologies, the ultimate whole-well ROP enhancement technology has been formed.

Due to impacts of tectonic activities and crustal stresses, deep low-permeability sandstone reservoir formations of Yanchang Formation in certain parts of the Ordos Basin contain well-developed fractures. To identify and assess these fractures, dipmeter logging conductivities and polar plate orientation plots were processed to generate direct mapping technique for detection of conductivity anomalies. Through comparison with electric imaging interpretation results, responses of various fractures were identified to further clarify application conditions of the technique. Through processing of actual data, it is determined that higher fracture apertures in the low-permeability sandstone formations may lead to more obvious features of high conductivity (low resistance) induced by drilling fluids. By way of conductivity anomaly detection, fractures with apertures over 45 μm can be identified directly. In addition, orientations, heights, dips and directions of maximum horizontal stresses in such formations can be determined accurately. As for fractures with apertures below 45 μm, the technique reveals lower accuracy in identification. Generally, the technique can be deployed in low-permeability reservoir formations with fractures developed to identify fractures and assess fracture parameters effectively.