Facies, potential and source rocks of petroliferous basins are three factors controlling the traps petroliferous property, they are necessary for reservoirs to trap hydrocarbons. Under the condition that they all met their own critical conditions of controlling reservoirs, the larger the facies-potential-sources coupling index, the higher the petroliferous property of the trap. Based on the mechanism for hydrocarbon accurnulation controlled by facies-potential-source coupling, physical simulation experiment was conducted. Studies showed that sources controlled the material origin of hydrocarbons, facies controlled the pore space to store hydrocarbons, and potential was the driving force for hydrocarbons to accumulate in traps. Only when they combined with each other, could hydrocarbon reservoirs form in basins. Furthermore, the larger the grain size difference between cap rocks outside the trap and the reservoirs inside the trap was, the easier it was for hydrocarbons to migrate from wall rocks of fine grains and low porosity to reservoir of coarse grains with high porosity. Meanwhile, its critical condition was that the ratio of grain size of reservoir to that of wall rocks was larger than 2 times, or the outside capillary force was more than 2 times larger than that inside the traps. At the same time, high petroliferous property of source rocks was better for hydrocarbons to accumulate in the traps|with hydrocarbon saturation of outer source rock reached or more than 5% as the critical condition.
Based on methods of rock thin section identification, fluid inclusion uniform temperature test, burial history and organic matter thermal history analysis, gypsum bed dehydration and clay mineral transformation analysis, it was shown that reservoirs in the Member 4 of the Shahejie Formation in Hexiwu structural zone had experienced alkaline→acidic→alkaline→weak acidic→weak alkaline diagenetic environment during their evolution process. Moreover, the diagenetic evolution sequences were compaction→feldspar dissolution and/or secondary enlargement of the quartz→carbonate cementation and/or anhydrite cementation→little carbonate cement dissolution→little pyrite cementation. According to the principle of inversion and back-stripping, the authors calculated each of the key diagenetic contributions on reservoir plane porosity quantitatively on the basis of casting thin sections with different diagenetic characteristics in the study area. Furthermore, the actural porosity was calculated by using the functional relationship with porosity. Finally, the evolution histories of actual porosity with different diagenetic characteristics have been established quantitatively combined with the mechanical compaction correction. In summary, from the matching relationship between porosity evolution and hydrocarbon accumulation period, in the first period of hydrocarbon accumulation, the reservoir physical properties were good with porosity up to about 20％. In the second period of hydrocarbon accumulation, the reservoir porosity rapidly decreased to about 7％ in the northern block as a result of intense carbonate cementation, and was not good for hydrocarbon accumulation. However, carbonate cementation in the southern block was weak, and most of the reservoir porosity is bigger than 10％. Therefore, the southern block was good for hydrocarbon accumulation, and became the favorable exploration target.
Nano-pores dominate the storage space of tight reservoirs and the connectivity of pore throat system is complex. The diameter of organic matter pores and intra-particle pores in marine shale with high maturity of southern China are 20～890 nm. The pores in terrestrial shale are organic matter pores and matrix pores, with diameter between 30～200 nm. The storage space in tight sandstone includes dissolution pores and micro-fractures with diameter in 10～200 μm. Nano-pores are composed by inter-particle pores and inter-crystal pores with diameter 70～400 nm. The calcite dissolution pores, inter-particle dissolution pores and micro-fractures dominate the nano-pores in tight limestones and the diameter is 50～500 nm. Reservoir digenetic modeling data indicates that total porosity in shale increases firstly and decreased later with the increase of|temperature and pressure. Four occurrences of oil in nano-pores have been distinguished and inter-particle pores are the most favorable for the oil accumulation. Moreover, due to the special characteristics of unconventional oil and gas, further work should be focused on the instrument improvement, technology innovation and evaluation parameter optimization.
The Fuyu Layer of Cretaceous Quantou Formation in northern Songliao Basin developed a set of low permeable tight sandstone reservoirs formed in river to shallow-water delta environments. Proven reserves in the Fuyu Layer was preserved in reservoir rocks with average porosity of 11. 8％ and average permeability of 2. 30×10-3 μm2, lithology reservoir was the main reservoir type. The remaining exploration targets of the Fuyu Layer were tight reservoir with porosity less than 10％ and permeability less than 1×10-3 μm2 . The main controlling factors of the tight oil accumulations in Fuyu Layer could be listed as follows: The mature source rocks controlled the distribution range of tight oil;the structural heights were the target areas for petroleum migration and accumulation;the NW fault belts controlled the petroleum enrichment areas; the channel sand bodies controlled the tight oil “sweet spots”. Based on the analogy method, the exploration potential for tight oil accumulations in Fuyu Layer was evaluated, and the preliminary result is about 13. 09×108 t tight oil to be proved, which would provide an important resource support for Daqing Oilfield.
Based on detail analysis of data of well logging, seismic records, outcrops and cores, the relationship between shale-member sedimentary facies and total organic carbons(TOC) of the Lower Silurian in Middle-Upper Yangtze area was discussed, and the favourable exploration directions were predicted. The Lower Silurian Longmaxi Formation consists of black shales interbedded with greenish-gray mudy siltstones, with the thickness of 20～268 m, in Middle-Upper Yangtze area, and two third-order stratigraphy sequences of SQ1 and SQ2, which distributed|stably could be|easily correlated, were distinguished from bottom to top within synchronous|stratigraphic framework. The transgressive systems tract(TST)of lower sequence SQ1 developed a set of organic-rich shale which is favorable for shale gas. During the depositional period of TST, the sedimentary pattern can be described as “abyss-area was open towards north, old lands were located in the east, south and west, deep shelf distributed extensively”, with the deep shelf area of 255 000 km2, and the average thickness of 35 m. Affected by the fall of sea-level, the shallowing of water took place, and lithofacies changed from black shales of deep shelf to laminated silty shales of middle and shallow shelf facies from the lower sequence to the upper sequence. The development of lithofacies was controlled by eustacy of sea-level, old lands, sea-bottom topography and sediment supply, and five lithofacies types including carbonaceous or siliceous or calcareous shales, laminated shales, biologic-stirred silty shales, laminated silty shales and interbeds of massive sandstones or shell limestones were developed. It was proved that the forming of organic-rich shales were intrinsically controlled by sedimentary facies, with deep shelf, middle shelf and lagoon generating high|content of TOC. So far, there were several wells of industry gas flow from the Longmaxi Formation, which had revealed well exploration prospect, for the Middle-Upper Yangtze area. It is synthetically concluded that zones of Fuling-Shishui-Renhuai, Weiyuan-Changning and western Hubei Province-eastern Sichuan Province are the main exploration directions and favorable targets of shale gas in the Lower Silurian of Middle-Upper Yangtze area.
The shale gas resource is abundant in China, but the exploration level is relatively low. High-precision liquid detection method based on seismic data is critical as there is few well data. Based on the seismic elastic wave equation, the liquid mobility factor formula is deduced and|the efficient exploration technique combined with high-precision time-frequency analysis algorithm is formed. The liquid mobility factor has a positive correlation with permeability, viscosity and density of reservoir. Based on the high-precision time-frequency analysis, the character of liquid is analysed, and the liquid mobility factor is calculated. Then, combined with well data, the range of factor in “sweet spot” area is guantified. The application in the Sichuan Basin demonstrates|the precision and efficiency of the method as the result is consistent with the test data of well. This method will play a significant guiding role in shale gas exploration.
Based on the core observation and thin section identification of 38 wells drilled through the Member 1 of Paleogene Funing Formation in Gaoyou sag, Subei Basin, it was discovered that the sedimentary facies was characterized by shallow delta and beach bar sedimentary features, which was different from that of previous study. The shallow delta facies could be divided into delta plain and delta front sub-facies. The former subfacies, contained distributary channel, mouth bar and subaqueous natural levee and the latter one included subaqueous distributary channel, subaqueous natural levee, bay between distributary channels and river mouth bar. In the period of lake level fluctuation, the distributary channels of shallow delta|stretched into the center of the lake basin to form several superimposed lobes. While adjacent to the edge of shallow delta, beach bars were distributed in ribbon shape along lacustrine shoreline by the transformation of lake wave and coastal current. Thus, a new type of sedimentary model named shallow delta and beach bar depositional model was set up.
Quaternary deep water contourites were studied based on seismic data in northern South China Sea(NSCS). Giant elongated drifts, confined drifts, slope sheeted drifts and sediment waves were found between about 1200m and 3000m water depth in NSCS. Giant drifts showed mounded morphology with moats on the landward flank. Confined drifts were developed in the negative relief among topography prominences, where were relative flat and with moats. Slope sheet drifts displayed with sheet morphology. Large scale of sediment waves were generated partially associated to drifts. When deep water contour current moved from northeastern to southwestern, because of relatively obvious topographical change and Coriolis force, helical contour current was produced and secondary circulation appeared in further and formed giant elongated drifts and confined drifts in the upper-middle slope. However, tabular contour current may produce slope sheet|drifts for flat and unrestricted environment in middle-low slope. This study|was not only helpful to improve the recognition of deep water contourite, but also was beneficial to serve the hydrocarbon exploration.
