川东北二叠系长兴组碳酸盐岩云化成储机制

Reservoir-forming mechanism of carbonate dolomitization of Permian Changxing Formation in northeastern Sichuan Basin

四川盆地是我国重要的含油气盆地,其中川东北二叠系长兴组碳酸盐岩是重要的产气地层。近年来发现的普光、龙岗、元坝等天然气田是重要的西气东输基地,对国家油气战略安全具有重要贡献作用。通过野外露头样品及钻井岩心分析,对川东北二叠系长兴组碳酸盐岩云化成储机制及主控因素进行了系统梳理。研究结果表明:①川东北二叠系长兴组气藏主要是一种白云岩气藏,生物礁灰岩储集能力较差与沥青充填有关,为气源层。野外露头考察发现礁盖白云岩具有更优的储集性能,但普光长兴组顶部的白云岩为一种非礁盖白云岩,揭示了长兴顶部的纯白云岩本身就是含气储层,与生物礁关系不大。②川东北长兴组白云岩储层的储集空间包括溶洞、晶间孔、晶内溶孔和晶间缝,为成岩过程中彻底云化而成。③川东北长兴组白云岩化存在5种成因机理:暴露淡水淋滤与蒸发泵流作用、渗透回流或盐水回流台地交代作用、埋藏成岩白云岩化、热液白云岩化、风化交代白云岩化,以前两种为主。表生和早期成岩的白云岩受沉积微相控制,不同区域具有不同的成因模式。川东北生物礁常常发生白云岩化作用,尤其是生物礁顶部,白云岩化后储集性能得到较大改善。④研究区长兴组中上部白云岩气藏的发现具有重要的理论意义和工业价值,尤其是长兴组顶部的白云岩自身具有良好的天然气储集能力,在封盖条件较好的广大区域均可形成工业气藏,川东北开江—梁平海槽台缘外(包括平昌海槽)长兴组也存在发育工业大气田的潜力。

Sichuan Basin is an important petroliferous basin in China,and carbonate rocks of Permian Changxing Formation in northeastern Sichuan Basin are important gas producing strata. The natural gas fields discovered in recent years such as Puguang,Longgang and Yuanba are important bases for West-to-East Gas Transmission,and play an important role in the national oil and gas strategic security. Based on the analysis of field outcrop samples and drilling cores,reservoir-forming mechanism of carbonate dolomitization and main controlling factors of Permian Changxing Formation in northeastern Sichuan Basin were analyzed. The results show that:(1)The gas reservoir of Permian Changxing Formation in northeastern Sichuan Basin is mainly dolomitic gas reservoir,and the poor reservoir capacity of reef limestone is related to asphalt filling,which is a gas source layer. The reef cap dolomite has more developed reservoir space according to field outcrop investigation. The dolomite at the top of Changxing Formation in Puguang gas field is a kind of non-reef-cap dolomite,which reveals that the pure dolomite at the top of Changxing Formation is a gas-bearing reservoir and has little relationship with biogenic reef.(2)The reservoir space of dolomitic reservoirs of Changxing Formation in northeastern Sichuan Basin includes karst caves,intercrystalline pores,intracrystalline dissolved pores and intracrystalline fracture,which are completely formed by dolomitization in the process of diagenesis.(3)There are five genetic mechanisms for dolomitization of Changxing Formation in northeastern Sichuan Basin:exposed fresh water leaching and evaporation pumping,seepage reflux or saline reflux platform metasomatism,buried diagenetic dolomitization,hydrothermal dolomitization and weathering metasomatic dolomitization,with the former two being dominant. Supergene and early diagenetic dolomites are controlled by sedimentary microfacies,and there are different genetic models in different regions. Dolomitization often occurs in reefs in northeastern Sichuan Basin,especially at the top of reefs. The reservoir properties have been greatly improved after dolomitization.(4)The discovery of dolomitic gas reservoirs at the middle and upper part of Changxing Formation in the study area is of great theoretical significance and industrial value. The dolomite at the top of Changxing Formation has good natural gas storage capacity,and industrial gas reservoirs can be formed as long as the corresponding conditions are available. Changxing Formation outside the platform margin of Kaijiang-Liangping trough(including Pingchang trough)has the potential of large industrial gas fields.