Taking the shales of the Longmaxi formation in the Sichuan basin and the Yanchang formation in the Ordos basin as examples, scanning electron microscopy, nuclear magnetic resonance, CT scanning, rock mechanics test and hydraulic fracturing experiments were carried out to investigate the differences in the shale microstructures between marine and continental facies, and then the influence of the differences on shale fracbility was analyzed, and the two kinds of shale were compared by considering fractal geometry and the theory of rock mechanics. According to the research results, the following findings are obtained: a.The continental shale (Yanchang formation) has a high content of clay minerals, accounting for about 45.3%, while the marine shale (Longmaxi formation) is dominated by brittle minerals such as quartz and feldspar, accounting for about 67.9%. b.The porosity of the marine shale is about twice that of the continental shale. c. The proportion of macropores in the marine shale is higher and the pore size distribution is wider, while mesopores and small pores in the continental shale occupy larger space, and natural fractures and beddings are developed. d.The average initial pressure of the continental shale is about 22.52% lower than that of the marine shale, mainly due to low mechanical strength caused by the high clay mineral content in the continental shale, but less effect from pore pressure. e.The major hydraulic fractures induced in the marine shale are transverse cracks that are basically symmetrical along the wellbore, while the natural fractures and beddings in the continental shale can easily capture hydraulic fractures, resulting in shear fractures perpendicular to the major fractures, and the open and connected beddings and weak surfaces are conducive to form a more complex fracture network in the continental shale. f.The three brittleness indicators of the continental shale are all lower than those of the marine shale, but the continental shale has advantages of lower initial pressure, better fracture network and better fracturing potential, so it is inaccurate to evaluate the compressibility of the continental shale by using a brittleness indicator. g.The low porosity, low permeability and high water sensitivity are not conducive to hydraulic fracturing stimulation to the continental shale reservoir.
The Ordos basin is the second largest sedimentary basin in China with abundant oil and gas resources and broad exploration prospects. Typical low-permeability-tight oil reservoirs are develpoed in the Triassic Yanchang formation in the basin,which are difficult to explore. Through continously geological researches on the Mesozoic oil reservoirs in the Ordos basin over the past 50 years,some theories about hydrocarbon accumulation in Jurassic reservoir groups,in large-scale lithologic reservoirs in inland depression lake basins and in continental shales have been formed. By virtue of three strategic shifts,four conventional hydrocarbon provinces with reserves exceeding 10×108 t and a successive zone with shale oil reserves of 20×108 t have been discovered. The proven oil reserves have increased by an average of over 3×108 t per year for 10 consecutive years. Thus,Changqing oilfield in Ordos basin has become an oil and gas province with the fastest increase in reserves and production in China and contributed 12.5% of China's annual oil production,which provides a reference for the petroleum exploration in other similar basins.
Less researches have been carried out on the microscopic pore characteristics of the Fengcheng formation shale in the Mahu sag. In order to determine the influence of pores on the occurrence and enrichment of shale oil, taking Well Maye-1 in the northwestern slope of the Mahu sag as a case, the characteristics of nano-pores and influencing factors in the continental shale were analyzed through X-ray diffraction, SEM, liquid nitrogen adsorption, high-pressure mercury intrusion, etc. The shale in the Fengcheng formation is mainly composed of felsic rock and carbonate rock. In the two kinds of rocks, the types, shapes and sizes of the pores are almost similar. There are inorganic pores, organic pores and micro-fractures, of which dissolution pores and micro-fractures are dominant. In addition, there are parallel plate-shaped fractures, wedge-shaped pores and ink-bottle-shaped pores. The pores are mainly small ones. The distribution of the pore size shows three peaks and the main peak ranges from 30 nm to 60 nm, and the pore connectivity is poor. The porosity of the felsic rock is higher than that of the carbonate rock, and macro-pores develop better in the felsic rock. So the felsic rock is more favorable for exploration. Quartz, feldspar and dolomite are controlling factors on pore development, and they have a balanced contribution to the pores of various sizes. Clay minerals are favorable for the development of micropores and small pores, but have a weaker impact on mesopores. Organic matter has a little effect on pore development. Both inorganic minerals and organic matter are favorable factors for the increase of shale porosity.
Dongdaohaizi sag is one of the important hydrocarbon-generating sags in the Junggar basin. Petroleum discoveries have been made in the Jurassic and Cretaceous in the uplift around the sag and in the Permian upper Wuerhe formation within the sag. Focusing on key wells with oil and gas breakthroughs in the upper Wuerhe formation within the sag, and combining with the changes in exploration ideas and theoretical understanding, the exploration of the upper Wuerhe formation in the study area is divided into three stages, namely, source-edge fault block exploration stage, in-sag fault block exploration stage, and above-source lithologic exploration stage. The changes in exploration ideas brought about the discovery of high-yield oil and gas reservoirs in the upper Wuerhe formation on the eastern slope of the sag. Also, insights in three aspects have been gained. First, the source rocks in the Pingdiquan formation have entered the stage of light hydrocarbon generation, contributing to the near-source hydrocarbon accumulation in the upper Wuerhe formation. Second, a large-scale retrograde fan delta sedimentary system is developed in the upper Wuerhe formation, where the fan delta front facies belt superimposed by the thick layers of lowstand system tract(LST) and the thin layers of transgressive system tract(TST) serves as favorable reservoirs, with a distribution area of 3 350 km2. Third, the hydrocarbon accumulation in the upper Wuerhe formation is characterized by sand enrichment in troughs, reserves controlled by facies belts, and production controlled by pores/fractures. Petroleum discoveries are concentrated in the eastern part, while no successful drilling result has been made in the western part of the sag. With these exploration ideas and geological understanding to guide the exploration deployment, there will be new discoveries of oil and gas in the Dongdaohaizi sag.
Bauxite gas reservoir is a kind of very rare unconventional gas reservoir recently discovered in the Ordos basin, and well logging evaluation plays an important role in its exploration and development. In the early well logging evaluation, bauxite was considered as the weathering crust caprock, but not as a reservoir, and there was no systematic well logging evaluation method suitable for the exploration and development of bauxite gas reservoirs. Based on the aluminous rocks in Taiyuan formation in the Longdong area, southwestern Ordos basin, the well logging evaluation method for bauxite gas reservoirs was studied from five aspects, that is, qualitative lithology identification, mineral composition, reservoir physical properties, quantitative calculation of gas-bearing properties and systematic summary of imaging model-pore structure characteristics. The well logging response for identifying aluminous rock formations was clarified and the aluminous rock identification chart by acoustic time-gamma ray was established. The porosity-permeability-saturation evaluation model for bauxite gas reservoirs was constructed through petrophysical experiments, and the criteria for identifying bauxite reservoir was proposed by combining micro-resistivity scanning imaging and nuclear magnetic resonance logging data. Finally, a well logging evaluation method for bauxite gas reservoirs was formed.
In this study, X-ray diffraction (XRD) and scanning electron microscope (SEM) were used to analyze the composition characteristics and genesis of clay minerals in the source rocks of the Lower Permian Fengcheng formation of the alkaline lacustrine facies in the Mahu sag, Junggar basin. The results show that the smectite and illite/smectite mixed layer has a higher content, but the illite content is lower, showing slow illitization of smectite. Considering the depositional background of the Fengcheng formation, it is concluded that the abnormal transformation of smectite is mainly affected by the volcanic rock, climate and diagenetic fluids in the source area. The intermediate-basic volcanic rock and the dry/wet alternating climate promoted the formation of smectite, while the alkaline water with CO2-3 as the primary anion inhibited the illitization of smectite.
In southwestern Tarim basin in the piedmont of western Kunlun Mountain, there are several thrust belts where multiple oil and gas fields have been found. Based on the fault-related fold theory and drilling and 2D seismic data, the Yingjisha anticline in the piedmont of southwestern Tarim basin is finely interpreted, where shallow detachment layers are discovered in the mudstone and gypsum mudstone of the Miocene Anju’an formation, and several shallow simple fault-bended folds are developed as well. It is concluded that the Yingjisha anticline is composed of shallow, middle-deep and deep structural intervals that are superimposed vertically. A wedge structure which contains multiple imbricate structures of thrust faults are developed in the middle-deep interval. Simple imbricate structures are in the deep interval. The growth strata and structural deformation styles indicated that the shallow structures were formed the earliest, followed by the middle-deep structures, and finally the deep structures. In short, there are multiple stages of episodic thrusts starting from the west Kunlun Mountain toward the Tarim basin, so the thrust structures in southwestern Tarim basin have undergone multiple stages of episodic activities. The structures developed in different periods are superimposed vertically, resulting in the present structural pattern.
To study the development characteristics and model of the Longwang Cave in the Cili area of Hunan province, a 3D model is constructed by using manual survey and 3D laser scanning technology to characterize the structure of the karst cave. The Longwang Cave can be divided into three sections. The western section is NEE-SWW trending and filled with chemical cements. The middle section is NNE-SSW trending and filled with collapsed cements. The eastern section is nearly EW trending, and filled with chemical cements and flowing water deposits. As for the filling degree, the middle section shows the highest, the western section is higher, and the eastern section is the lowest. Controlled by a NEE-SWW trending fracture zone, the Longwang Cave is a typical fracture-controlled karst cave which has undergone four development stages. First, a NEE-SWW fracture zone was formed along the Sanguansi syncline. Then the rock was dissolved along the fracture zone by karst water from west to east, and an initial karst cave formed. Second, the initial karst cave was dislocated by NNW-SSE trending strike-slip thrust faults and then differential dissolution occured, that is, the western section was dissolved to a higher degree than the middle and eastern sections. Third, due to different numbers and scales of fractures, differential dissolution made the carst caves in the western section further expand, and the karst caves in the eastern section gradually connected together. The dissolution of the western section was higher than that of the eastern section. Finally, the middle section inherited the previous dissolution characteristics, so it is weaker than the eastern and western sections in dissolution. After the four stages above, the Longwang Cave shows its present structure.
Major breakthroughs to oil and gas exploration have been made in the northern slope of Mahu sag, but little exploration effect has been obtained in the adjacent western slope. According to the data from cores, thin sections and scanning electron microscope, the petrological characteristics and diagenesis of the Baikouquan reservoirs in the northern and western slopes are compared and analyzed. It is found that the reservoir physical properties and controlling factors are very different, and compaction, cementation and dissolution are fundamental causes for the different reservoir physical properties. The western slope is better than the northern slope in oil and gas resources. Favorable reservoirs are distributed in the deep of the western slope, and future exploration should focus on the two wings of the fan delta and the area around Well Mazhong-1.
The southern margin of the Junggar basin is a typical petroliferous area with high or ultra-high pressure. To understand the oil and gas accumulation law and predict favorable exploration zones, it is important to clarify how the overpressure develops and how the oil and gas are preserved at strong overpressure. According to the overpressure characteristics and the exploration results from Well Gaotan 1, this study analyzed the overpressure causes in Well Gaotan 1, and then predicted the height of the hydrocarbon column that is dynamically closed by the hydraulic fractures in the caprock in the overpressure system. It’s found that there are multiple factors causing the overpressure in the Cretaceous Qingshuihe formation in Well Gaotan 1: tectonic compression accounts for 51.03%, pressure transmission accounts for 14.94% and undercompaction accounts for 34.03%; and the tectonic thrust and lateral compression stress during the Himalayan movement are major inducements of the abnormally high pressure in the deep formation. In the Cretaceous Qingshuihe formation in Well Gaotan 1, the mudstone caprock is thick, the displacement pressure is high and the sealing ability is strong. Therefore, hydraulic fractures in the caprock and the re-slipping of the pre-existing faults in the overpressure system dynamically control the maximum overpressure that the caprock can withstand and the maximum height of the hydrocarbon column that can be closed. The Cretaceous Qingshuihe formation and the Jurassic Toutunhe formation in Well Gaotan 1 are two independent pressure systems. The pressure coefficient of the Qingshuihe formation is 2.32, close to the critical pressure for the sliding of the pre-existing fault, so it is estimated that the maximum height of hydrocarbon column that can be dynamically closed before the burst of the caprocks is 200 m. The Middle and Upper Jurassic strata in the Gaoquan anticline are the next exploration target, the channel-delta front sandbodies may have high-quality reservoirs, and future exploration may focus on structural and lithological reservoirs.
In order to identify the pore characteristics of the continental shale in the seventh member of Yanchang formation (Chang 7 member) in southeastern Ordos basin, the shale cores taken from pure shale and silty laminae of Chang 7 member were analyzed for pore characteristics with the aid of experimental methods such as SEM, cast slice, gas adsorption method, and mercury intrusion, and the factors that may affect pore development were discussed. The results show that pure shale mainly contains clay mineral intergranular pores and organic pores, and silty laminae holds intergranular pores and intragranular dissolved pores. Compared with pure shale, silty laminae has mesopores-macropores with larger porosity, pore diameter and pore volume. In pure shale, the development of pores is mainly controlled by the contents of rigid particle and organic matter. In silty laminate, the controlling factors for shale pore development are mainly the preservation conditions. For instance, the enrichment of rigid particles such as quartz and feldspar is conducive to pore preservation, acidic fluid can corrode feldspar to create more pores, and liquid hydrocarbons can wrap minerals to inhibit cementation. The shale in silty laminae is superior to pure shale with respect to pore structure and physical properties. Thus, the silty laminae zones should be paid more attention in shale oil/gas exploration.
In order to improve the drilling rate of pay zones in the Ma55 gas reservoir in eastern Sulige gas field, Ordos basin, using the drilling, logging, core and gas production testing data, and analyzing the main factors controlling pay zones such as sedimentary microfacies, diagenesis and paleotopography, the distribution law of dolomite in the Ma55 gas reservoir was clarified. Moreover, the reservoirs were comprehensively classified and evaluated, and the favorable gas enrichment areas in the Ma55 gas reservoir were selected. The research results show that the pay zones in the Ma55 reservoir are distributed as “lens” in local areas, with poor continuity. The most favorable reservoir rocks are granular dolomite and coarse powder crystalline dolomite, and the main storage space consists of intergranular pores, intergranular dissolved pores and structural fractures. Sedimentary facies and diagenesis are the main controlling factors of the Ma55 reservoir, and the grain beach is the most favorable sedimentary microfacies of the Ma55 dolomite. The pay zones are mainly controlled by quasi-contemporaneous dolomitization and buried dolomite diagenesis. The paleoslope on the relatively high position is a favorable area for the development of the Ma55 gas reservoir.
As an important reservoir for storing oil and gas,the sand bodies in delta front are found with enormous petroleum exploration potential. However,there are few studies on architecture of sand bodies in shallow delta front through field outcrops. Guided by sedimentology and reservoir architecture theories,the outcrop observation and sampling was combined with the results of laboratory experiments and statistical analysis to clarify the sedimentary characteristics and sand body architecture of Chang 9 member in the Shiwanghe section in Yichuan,Ordos basin. The results show that during the deposition,the Chang 9 member in Shiwanghe section lied in a warm and humid environment,especially an oxidation to weak-reduction transitional freshwater environment that was not obviously stratified,and shallow delta front subfacies was mainly developed,including microfacies such as underwater distributary channel,estuary bar,sheet sand and interdistributary bay. The single sand bodies of shallow delta front in Chang 9 member in the study area can be divided into two vertical stacking styles such as non-connected and connected,and two lateral contact styles such as butted and cut-stacked. The accommodation growth rate and sediment supply rate jointly controlled by terrain slope and lake level rise/fall are important factors affecting the spatial development style of the composite sand bodies. The gentle slope allows the channels to incise weakly and present the characteristics of plane intersection. The rise of lake level and the decrease of source supply increase the ratio of accommodation growth rate to sediment supply rate,which may lead to the weakening of sand body connectivity. The architecture models of sand bodies like river-river cut stacking and river-bar cut stacking are favorable for hydrocarbon accumulation.
The Fengcheng formation in the Mahu sag is an alkaline lake sediment，and is divided into Feng 1 member，Feng 2 member and Feng 3 member from bottom to top. In the Fengcheng formation，the lithology vertically changes rapidly，the mineral composition is complex，and the organic-rich shale source is integrated with the shale reservoir. The formation bears oil universally，but the sweet spots are scattered. The results of formation testing for single layers are not satisfied，showing an unclear production potential. According to core slices and geochemical analyses，the Fengcheng formation in the Mahu sag is dominated by lamellar silty shale intercalated with dolomite，which are mainly composed of terrigenous clastic minerals and carbonate minerals. With the variation of burial depth，the pore volume changes consistently with the variation of surface area of pores，and the pore volume is mainly contributed by macropores (pore diameter > 50 nm). The source rock is dominated by Type Ⅱ organic matter，and the vitrinite reflectance ranges from 0.85% to 1.40%，indicating a peak oil generation period. There are many shear fractures with middle to high angles in the Feng 2 member，and shear fractures with middle to high angles and structural fractures with low angles in the Feng 3 member，whose formation and development degree are controlled by lithology，mineral composition，rock mechanical properties，etc. Based on the characteristics of lithologic assemblage，reservoir property and oil-bearing property，four relatively concentrated sweet spots have been identified. When performing multi-interval formation testing and production testing in vertical wells，it is necessary to select sweet spots with good oil content and more fractures to conduct geological research and geology-engineering integration technology research，and to perform production improvement tests in horizontal wells，so as to realize comprehensive breakthrough for shale oil exploration and development in the Fengcheng formation in the study area
The Ordovician carbonate reservoir in Halahatang oilfield is the primary contributor to high and stable production of Tarim Oilfield Company. However, it has been difficult to study its forming mechanism, forming period, and oil and gas distribution. Based on geophysical data, core and slice observation, analysis of geochemical and production data, etc., the characteristics of the hydrothermal karst in the Halahatang area have been studied, the forming period of the hydrothermal karst has been determined, and the controls of the active faults formed in the Hercynian movement and the strike-slip faults formed in the Caledonian movement on the distribution of the hydrothermal karst and hydrocarbon are clarified. The study results show that magmatic activities, active faults in the Hercynian movement period, and hydrocarbon accumulation in hydrothermal karst mainly occurred in the Permian, and they are similar in timing and successive in origin. There are differences between the Hercynian active faults and the Caledonian strike-slip faults in terms of mechanical properties, occurrence, scale, distribution, and controls on hydrothermal karst. Hydrocarbon can effectively accumulate in the hydrothermal karst on the strike-slip faults in the Caledonian movement period, while hydrocarbon can not or poorly accumulate in the hydrothermal karst on the active faults in the Hercynian movement period due to the influences of the openning and intermitting of faults during the hydrocarbon accumulation period. As a result, the distribution of the hydrocarbon in the Ordovician strata in the east of the Halahatang area greatly differs from that in the west. During petroleum exploration and well allocation, it is necessary to differ the Caledonian strike-slip faults from the Hercynian active faults, and then take the hydrothermal karst on the strike-slip faults as the exploration target.
The Shunbei No. 4 strike-slip fault zone which is located in the Shuntuoguole low uplift of the Tarim basin and extends northward to the Shaya uplift is characterized by deep burial，horizontal segmentation，vertical stratification，multi-stage activities，and complex structure. Through the interpretation of high-quality 3D seismic data from the Shunbei No. 4 strike-slip fault zone，the stratification，segmentation，staging，activity and favorable area evaluation of the fault zone were carried out. The results show that the Shunbei No. 4 strike-slip fault zone has a 4-layer structure in the Paleozoic，roughly bounded by the top of the Middle Ordovician，above which echelon faults are found and below which high-steep strike-slip faults are developed. The strike-slip fault zone is visibly segmented into the northern segment，the middle segment，and the southern segment according to the strike，showing an overall characteristic of compressed in south and extended in north. In the Paleozoic，the strike-slip fault zone successively experienced four periods of activity，namely，EpisodeⅠof the middle Caledonian，Episode Ⅲ of the middle Caledonian，late Caledonian，and Hercynian. By combining the main controlling factors (e.g. source-reservoir connectivity，reservoir size，and late adjustment) for hydrocarbon enrichment and accumulation in the Shunbei area，the favorable areas in the Shunbei No. 4 strike-slip fault zone were evaluated. Multiple favorable areas have been identified and then verified by actual drilling.
The Turpan-Hami basin has great potential of oil and gas resources in the Lower Jurassic strata, with a large quantity of remaining resources. The discovered oil and gas reservoirs are mainly distributed in the positive structural belts around the Shengbei and Qiudong subsags in the Taibei sag, and they are primarily structural reservoirs. Less researches on the oil and gas resources in the hinterland of the subsags have been performed. Based on the dissection of known reservoirs, a systematic study was carried out on the depositional system, source rock, reservoir rock and accumulation conditions of three major hydrocarbon-rich subsags in the Taibei sag. The results show that the coal-measure source rocks are widely developed in the Shuixigou group in the Taibei sag and are in broad contact with the braided river delta sandstones, which is conducive to the formation of near-source tight sandstone gas reservoirs. There are two types of tight sandstone gas reservoirs in the Lower Jurassic, namely, trap-type and continuous-type. The hinterlands of the subsags are favorable areas for the formation of continuous-type tight sandstone gas reservoirs. Therefore, the exploration should be switched from the source-edge positive structure to the hydrocarbon-rich subsag, and from the above-source conventional oil reservoirs to the in/near-source tight sandstone gas reservoirs. The hinterlands of the Shengbei and Qiudong subsags have the conditions to form large gas reservoirs, so they are favorable areas for exploring near-source tight sandstone gas reservoirs in the lower Jurassic.
In order to understand the mechanism that the karstification during the Episode Ⅱof the Middle Caledonian movement controlled the development of karst caves in the Upper Ordovician coverage area in the western slope area of Tahe oilfield, the paleo-hydrogeomorphic and underground karst cave system of the Episode Ⅱ of the Middle Caledonian movement were precisely described using different methods. The results show that the karst platform is dominated by karst hills and depressions, the surface water system is dendritic, and the underground river-cave system is developed, forming an “open” underground river-karst system. The karst slope is dominated by hills and valleys, and deep incised valleys are developed because of strong vertical erosion, forming a “downward” buried fault-controlled karst system. The karst basin in the southern part of the platform margin is flat and the surface runoff is underdeveloped, with weak vertical erosion, but mainly horizontal dissolution, forming a “rising” buried fault-controlled karst system. Based on the paleo-hydrogeomorphic characteristics, the development model of karst caves under the control of special hydrogeomorphology of the Episode Ⅱof the Middle Caledonian movement was established for the Lianglitage formation coverage area in the western slope area of Tahe oilfield, which provides a geological basis for subsequent rolling development.
The Carboniferous in southern Chepaizi oilfield is mainly composed of amygdaloidal basalt, basaltic andesitic agglomerate, andesitic breccia tuff and a small amount of basaltic andesitic breccia tuff. In order to determine the geological age of these rocks, isotopic and biofossil analyses were carried out, and then the analytical results were compared with the Carboniferous rocks in the outcrop area in the basin margin. It is found that there are abundant sporopollen fossils in the bottom of the Carboniferous sandstone in Well C47, and the zircon U-Pb age of the rhyolitic breccia-bearing vitric tuff from Well C68 is 314.6±2.1 Ma, suggesting that both the geological age and isotopic age of the volcanic rock are Late Carboniferous, which are comparable to the Hala’alat formation in the piedmont at the northwestern margin of the Junggar basin. The volcanic rocks are generally calc-alkali-tholeiitic series and relatively rich in Al2O3, with a weak positive Eu anomaly. Moreover, the volcanic rocks are strongly short of high field-strength elements such as Nb, Ta, and Hf, and relatively rich in large ion lithophile elements such as Ba, Rb, and K. There exists an obvious Nb-Ta trough, and the magma source area may be the depleted mantle of spinel peridotite because of metasomatism, and formed in the subduction-related island arc tectonic environment. For the well blocks in southern Chepaizi oilfield, the Carboniferous in the northwest wall of the large fault can be compared with the Aladeyikesai formation, and the volcanic rock formation in the southeast wall can be correlated with the Hala’alat formation in the piedmont of the northwest margin.
In the Qingyang gas field, Ordos basin, which is a typical tight sandstone gas field, the major pay zone is the first member of the Permian Shanxi formation. Its sedimentary sand bodies change rapidly with small thickness, making the prediction of reservoir distribution difficult, which restricts the productivity construction of the gas field. In this paper, sedimentary sand body characterization and thin layer prediction were carried out using logging-seismic combination, the main factors controlling reservoir development were discussed, and favorable reservoir distribution areas were identified. The results show that the paleogeomorphology and paleo-flow direction jointly controlled the distribution of delta sand bodies, the underwater distributary channel sand bodies with developed dissolution facies are the most favorable reservoirs, and the local micro-amplitude nose uplift structure is the natural gas enrichment area. Based on the factors such as sand body distribution, reservoir physical properties, diagenesis and structural characteristics, a set of standards for classifying reservoirs in the study area was established, by which two Type I reservoir enrichment areas were defined in the southern and central parts of the study area.
