四川盆地页岩气成藏地质条件

GEOLOGICAL CONDITION OF SHALE GAS ACCUMULATION IN SICHUAN BASIN

与传统上的"泥页岩裂缝气"并不完全相同,页岩气是主体上以吸附相和游离相同时赋存于泥岩及页岩地层中的天然气。四川盆地经历了克拉通和前陆盆地演化过程中复杂的构造变动,形成了与美国典型页岩气盆地相似的构造演化特点和地质条件,其中的古生界页岩不仅是盆地内常规气藏的烃源岩,而且还是页岩气成藏及勘探的主要对象,目前已发现了页岩气存在的大量证据。根据演化及勘探地质特点,四川盆地非常规天然气具有两分格局,东南部以页岩气为主而西北部以根缘气为主,古生界主体发育页岩气而中生界主体发育根缘气。川东和川南地区(包括川西南)古生界生气页岩发育厚度大、有机质含量高、埋藏深度小,下寒武统和下志留统具有良好的页岩气成藏及勘探地质条件;川中地区同时发育中、古生界烃源岩,上三叠统、下志留统和下寒武统可作为页岩气勘探的有利层位;川西中生界泥/页岩常与致密砂岩形成频繁互层并产生具有砂岩底部含气特点的根缘气,整体上存在着页岩气发育和勘探的远景条件,局部埋藏相对较浅的高碳泥/页岩是页岩气勘探的基本对象。

Distinguished by what from traditional conception of fissured shale gas, shale gas is concerned to the gas accumulated within the shales and/or mudstones by free and adsorbed gas. The Sichuan Basin experienced complex tectonic movements in process of the evolution from Craton basin to foreland basin. As a result, The Sichuan Basin is of the similar tectonic characteristics and geological conditions for shale gas with the typical shale gas basins in America, where the Paleozoic shale is not only the source rocks for conventional gas reservoirs but also the main objects for shale gas accumulation and exploration. At present, a large number of evidences of shale gas have been found in the Sichuan Basin. According to the structural and evolutional characteristics of the Sichuan Basin, the unconventional gas is distributed by two distinguished patterns: in place, shale gas is mainly in the southeastern basin while the source-contacting gas in the northwestern basin; in time, shale gas accumulated in the Paleozoic Erathem while source-contacting gas in the Mesozoic Erathem. In North and South Sichuan Basin, including Southwestern Sichuan Basin, there developed Paleozoic Erathem gas bearing shales with huge thickness, high contents of organic matters and shallow buried depths. Lower Cambrian and Lower Silurian Series are of good geological conditions for shale gas accumulation and exploration. In Central Sichuan Basin there developed both Mesozoic and Paleozoic Erathems of source rocks, in which Upper Triassic Series is the favorable horizon for shale gas exploration. In West Sichuan Basin the gas bearing shales is mainly Mesozoic Erathem, in which shales and tight sands interbed frequently to form the source-contacting gas with the characteristics of gas accumulated on the bottom of tight sands. There exist perspective conditions for exploration and exploitation of shale gas in West Sichuan Basin, especially the high carbon shales and/or mudstones with relatively shallower depths.