四川盆地高石梯—磨溪地区震旦系—寒武系大型气藏特征与聚集模式

四川盆地震旦系—寒武系具有良好的油气成藏条件,但由于地层时代早、经历构造运动次数多,大型气田勘探难度增大。自1964年发现威远震旦系大型气田后,历经49年的艰苦探索,于2013年在磨溪地区寒武系龙王庙组发现了中国迄今为止单个规模最大的整装特大型气田——安岳气田,探明天然气地质储量4 404×108 m3,且高石梯—磨溪地区震旦系灯影组和寒武系龙王庙组气藏的三级储量之和超过万亿立方米。天然气组分、轻烃、储层沥青丰度等证据均表明这些天然气主要为原油裂解型干气,甲烷含量为82.65%~97.35%,乙烷含量为0.01%~0.29%;低氮(0.44%~6.13%)、低氦(0.01%~0.06%),中—低含硫化氢为主(0.62~61.11g/m3)。气藏压力由震旦系的常压(压力系数为1.07~1.13)逐渐增高至寒武系龙王庙组的高压(压力系数为1.53~1.70),气藏温度137.5~163.0℃;气藏圈闭类型包括构造型、构造-地层和构造-岩性复合型。桐湾期大型继承性稳定古隆起、大面积分布的古老烃源岩、大面积孔洞型优质储层、大型古油藏裂解及良好保存条件的有效配置造就了震旦系—寒武系天然气的规模富集。根据古油藏原油裂解之前的古构造格局、沥青丰度及现今气藏分布特点,将裂解气藏聚集类型划分为聚集型、半聚半散型和分散型3类。上述认识对四川盆地震旦系—寒武系天然气勘探领域的拓展具有重要指导意义。

Characteristics and accumulation modes of large gas reservoirs in Sinian-Cambrian of Gaoshiti-Moxi region,Sichuan Basin

The Sinian-Cambrian Formation of Sichuan Basin has favorable hydrocarbon accumulation conditions, but the difficulty in exploration of large gas field is increased due to old strata and multiple tectonic movements. After 49 years of arduous exploration since the Weiyuan Sinian large gas field was found in 1964, the largest mono-block gas field (Anyue gas field) was discovered in Cambrian Longwangmiao Formation of Moxi Region in 2013, with proven geological gas reserves of 4 404×10⁸m³. The proved, probable and possible reserves totally exceed trillion cubic meters in Sinian Dengying Formation and Cambrian Longwangmiao Formation, Gaoshiti-Moxi Region. The gas components, light hydrocarbon, reservoir bitumen abundance and other evidences prove that the natural gas is dominantly constituted of oil-cracking dry gas, including methane (82.65%-97.35%), ethane (0.01%-0.29%), low nitrogen (0.44%-6.13%), low helium(0.015%-0.06%), and medium-low hydrogen sulphide(0.62-61.11 g/m³). Gas reservoir pressure is gradually increased from Sinian ordinary pressure (1.07-1.13) to high pressure (1.53-1.70) in Cambrian Longwangmiao Formation. The temperature of gas reservoir is 137.5-163℃. Gas reservoir traps are divided into three categories, i.e., structural trap, stratigraphic structural combination trap and lithologic structural combination trap. The large-scale enrichment of Sinian-Cambrian natural gas is resulted from the large stable inherited paleo-uplift during Tongwan tectonization, wide distribution of ancient hydrocarbon source rocks, vast porous high-quality reservoirs, crude oil cracking of large paleo-reservoir and favorable preservation conditions. According to the paleo-structure pattern and bitumen abundance before crude oil cracking, and distribution characteristics of current gas reservoirs, the accumulation patterns of cracking gas reservoir can be classified into three types, i.e., aggregation type, semi-aggregation and semi-dispersion type and dispersion type. The above understanding will play an important guiding role in the exploration of Sinian-Cambrian natural gas in Sichuan Basin.