四川盆地页岩气藏和连续型-非连续型气藏基本特征

四川盆地是一个多旋回叠合盆地,发育了两套海相优质烃源岩(下寒武统牛蹄塘组页岩,下志留统龙马溪组页岩),其有机质成熟度高(R_o=1.5%～6%),绝大多数地区普遍不利于页岩气藏的形成;另发育两套海陆过渡相(上二叠统龙潭组和上三叠统须家河组)优质烃源岩,其成熟度相对较低(R_o=0.7%～3%),在川西南-川南地区具有较有利的页岩气勘探前景.四川盆地页岩烃源岩排烃效率高, 为大规模油气藏提供了烃源;多存在晚期快速隆升调整过程, 为吸附气的解吸创造了条件.页岩气藏经历了早期地质条件优越、生物气高效成藏,中期深埋地腹、原油裂解气快速成藏,晚期快速隆升、脱溶气和解吸气调整成藏等过程.四川叠合盆地油气藏具有多样性特征, 发育了从非连续型到连续型完整序列的油气藏类型.典型的非连续型气藏主要是指构造圈闭(以及部分构造-岩性圈闭)气藏,其圈闭相对独立,非连续分布, 储集空间类型以孔隙型为主.震旦系威远气田、川东石炭系气田群及川东北下三叠统飞仙关组气藏群等具有相对典型的非连续型气藏特征.非连续型-连续型过渡气藏介于连续型气藏与非连续型气藏之间,呈分散状或连续状分布, 圈闭类型多以复合圈闭为主,可发育有裂缝圈闭(川东南二叠系阳新统气藏)及岩性圈闭(上三叠统须家河组气藏).储集空间以裂缝型或次生溶孔为主,非均质性强,普遍存在异常高压.四川盆地连续型气藏应以页岩气藏为主, 但现今未有成功勘探开发页岩气藏的实例.四川叠合盆地具有多旋回构造-沉积演化、优质烃源岩分布的区域性、储层的非均质性和天然气的活动性,及油气成藏经历了生物气、吸附气、裂解气、脱溶气和解吸气演替等多样性特征.因此,四川盆地的油气勘探和研究应形成常规与非常规、连续型与非连续型、原生与次生油气藏的立体勘探和研究局面, 尤其应加强非连续型-连续型过渡油气藏特征、形成机理和分布规律的研究.

Basic characteristics of shale and continuous-discontinuous transition gas reservoirs in Sichuan Basin,China

Sichuan Basin is a polycyclic superimposed basin. There are two sets of high qualitative marine source rocks (Lower Cambrian Niutitang Formation shale and Lower Silurian Longmaxi Formation shale) with high maturity (R_o=1.5%～6%). Most areas of the basin are not favorable to Niutitang Formation shale and Longmaxi Formation shale gas accumulation. There exist two sets of transitional (marine-nonmarine) type high quality source rocks (Upper Permian Longtan Formation; Upper Triassic Xujiahe Formation) as well, with relatively low maturity (R_o =0.7%～3%). There will be a more favorable prospect area for the Longtan Formation and Xujiahe Formation shale gas exploration in the southwest and south of Sichuan Basin. The hydrocarbon expulsion efficiency of the shale source rocks in Sichuan Basin is high and the large scale of conventional gas fields has been formed. Shale gas reservoirs had superior geological conditions in the early stage and went through the biogenic gas efficient accumulation, rapid oil-cracking gas accumulation in the middle deep buried stage, accumulation and adjustment of de-dissolved and desorption gas in the late rapid uplifting stage, and so on. There are various oil & gas reservoirs in Sichuan Basin including continuous, discontinuous and transitional gas pools. Typical discontinuous gas reservoirs are structural trap and structural-lithologic trap gas reservoirs with relatively independent traps and discontinuous distribution and pore-type reservoir rocks. The examples include the Weiyuan Sinian System gas field in South Sichuan, Carboniferious System gas field groups in East Sichuan, Lower Triassic Feixianguan Formation gas field groups in Northeast Sichuan, and so on. The transitional reservoirs have dominant composite traps with fractured traps (Lower Permian Yangxin series gas reservoirs in Southeast Sichuan) and lithologic traps (Upper Triassic Xujiahe Formation gas reservoirs). Main reservoir pore spaces of the transition gas pools are fractures or secondary pores with strong heterogeneity and general overpressure. The distribution of transition gas reservoirs was scattered or continued. Shale gas reservoirs are typical continuous reservoirs, but there has not a successful exploration and development case in Sichuan Basin so far. Sichuan Basin has gone through multiple tectonic-sedimentary evolutions, resulting in regional distribution of high quality hydrocarbon sources, reservoir heterogeneity and gas activity. The hydrocarbon reservoirs have experienced successive and diverse processes, such as biogenic gas, adsorbed gas, oil-cracking gas, desolution gas and desorption gas. Therefore, the oil & gas exploration and research in Sichuan Basin should form dimensional exploration and research strategy, including conventional and unconventional, continuous and discontinuous, primary and secondary reservoirs, especially strengthening the study of characteristics on the formation mechanism and distribution of transitional (continuous-discontinuous) gas reservoirs.