Chishui And Adjacent Areas Triassic Xujiahe Deposition And Gas-bearing Characteristics Are Discussed

After referring & reading of many references, careful observation of outcrops, earnest study of spot data and correlative indoor analysis & test, the author studied gas-bearing characters of Upper Triassic Xujiahe Formation (T₃x) in Chishui Area, such as stratigraphic & sedimentary features, reservoir features, source rock features and original features of nature gas and so on. Then the author primarily discussed the characteristics of gas distribution and reservoiring. Meanwhile, the author analyzed character differences of T₃x between Chishui Area and other parts of Sichuan Basin, from several aspects. Finally some advices were advanced in the article, so that following study and petroleum exploration work could be carried out more effectively.In the study of stratigraphic & sedimentary features, based on former researches and investigation of working area, the author summarized the stratum distributing rules and sedimentary facies spreading characters of Upper Triassic in Sichuan Basin. Upper Triassic as a whole show a trend that is becoming thicker to northwest and thinner to southeast in Sichuan Basin. In west and middle area of the Basin, Upper Triassic could approximately be divided into two parts, "the lower basin of T₃x" and "the upper basin of T₃x", which were developed in different tectonic and sedimentary basement. "The lower basin of T₃x" are partly or all lack in east and southeast of the Basin, and dichotomy feature of Upper Triassic (or T₃x) is hardly exist. Ma'antang Formation and Xiaotangzi Formation of Upper Triassic are not developed in Chishui Area, what's more T₃x is only corresponding with "the upper basin of T₃x" or the strata up T₃x in west of the Basin. Compared with west and middle area of the Basin, sediments with relatively small size, such as dark mud-shale and coal, are less by far; sediments with relatively large size, such as sand, occupy comparatively more amount of the whole formation. This is controlled by paleogeography environment of T₃x. T₃x is mainly (sub-)deep lake facies with alluvial-submarine fan facies interbeded, and it is mainly shallow lake or lake delta facies, however it is mainly fluvial facies. Sediments of T₃x in working area are mainly sandstones interbeded few mudstones, and the mature degrees of structure & compositions of the sandstones are neither high. Obviously, they are fluvial sediments, and the river type changes frequently, mostly with two types which are braided river and meandering river, and the sediments of braided river are more by far in the whole formation.For the study of reservoir characters, according to the observation of outcrops and core, the author discussed the porosity & permeability features and their controlling factors of T₃x reservoirs in Chishui Area, and compared these reservoir characters withother parts of Sichuan Basin. General characters of T3X reservoirs in working area were advanced, including tight even super-tight sandstone type, especially low porosity & permeability features, fracture-pore reservoir type and the controlling factors and that the exist of fractures and secondary solution pores controlled promising reservoirs. It also was pointed out that fractures were always developed with secondary solution pores together, so where tectonic fractures are found frequently is also the developing place of promising reservoirs.In the source characteristic research, in terms of outdoor investigation and indoor analysis & test, the author understood distributing features and hydrocarbon-generating ability of T3X source rock fully, and compared the characters with west of the Basin. It is advanced that the dark shale and coal of T3X have hydrocarbon-generating ability in some degree, that the source belongs to III-kerogen and that the natural gas generated is coal-related gas. However, generally speaking, the source conditions of T3X in working area are not better than west or middle area of the Basin, both hydrocarbon-generating amount and hydrocarbon-generating density are less than other place of the Basin; the reasons lie in thinner source rock and worse coal-generating condition.According to the geochemistry characteristic analysis, gas from T3X has not typical characters of coal-related gas, in working area, so it has little relation with T3X source; considering constitute and carbon isotope characteristics of gas, it is generally consistent with the gases from lower marine facies strata, which were came from Permian and Silurian. Thus gas of T3X should mainly come from lower strata. The typical gas pool is mixed type pool, and it has the character of multistage migrating and reservoiring.