江汉盆地潜江凹陷古近系潜江组白云岩成因

潜江凹陷位于江汉盆地,其古近系潜江组以含有由富有机质白云岩和蒸发岩组成的193个韵律层为特征。然而,潜江凹陷古近系潜江组白云石的成因一直存在争议,限制了进一步对非常规油气的勘探工作。根据盐间层岩相与矿物特征研究潜江组盐间层富白云石岩相的沉积成因。根据沉积特征观测(岩心和薄片)和矿物组分数据,可识别出3种主要的岩相结合:薄层-纹层状泥晶白云岩、纹层状含云泥晶灰岩和夹钙芒硝层状泥晶白云岩。研究发现前两种岩相组合纹层特征明显,可见有机质纹层与无机纹层交互现象,说明形成于水体较深的还原环境。白云石与方解石晶体含有有机质降解孔,且白云石与黄铁矿和有机质薄膜共生,说明其成因与微生物诱导活动有关。浮游微生物和底栖藻分别控制了方解石和白云石的沉淀。这些岩相组合含有较高的有机质含量。最后一种岩相组合具有较高的蒸发矿物含量和陆源碎屑物质,说明其形成于相对浅水环境。高盐度和强烈陆源输入不利于有机质的保存。此次研究发现盐湖内白云石的形成受微生物活动和蒸发环境控制,不同的成因对岩相沉积特征和有机质富集产生重要影响。

Origin of Qianjiang Formation dolostone in Qianjiang Sag,Jianghan Basin

The Paleogene Qianjiang Formation of Qianjiang Sag in central Jianghan Basin is characterized by 193 cyclothems composed of alternating organic-rich dolomites and evaporites. However, the origin of dolomites in the Qianjiang Formation has always been controversial, limiting the further exploration of unconventional resources. The main objective of the study lies in the origin of inter-salt dolomite-rich lithofacies in the formation through the analyses of sedimentary characteristics of inter-salt lithofacies and minerals. Three main lithofacies combinations are identified based on the observation of sedimentary features (in terms of core and thin section) and mineral composition, that is, the thin-bedded-to-lamellar dolomicrite, the lamellar marlstone bearing dolomite, and the lamellar dolomicrite interbedded with glauberite. The first two combinations show obvious laminated characteristics with alternating organic and inorganic lamellars, indicating deposition in a relatively deep and reducing environment. The calcite and dolomite crystals within these two combinations have pores resulted from organic degradation, and the dolomite crystals coexist with organic films and pyrite crystals, both of which just indicate that the origin of the two combinations may have something to do with microbial activities. Planktons and phytobenthos serve to control the precipitation of calcite and dolomite minerals respectively. Therefore, the two lithofacies combinations feature a higher content of organic matters. The third combination contains more evaporitic minerals and terrigenous clasts, indicating deposition in relatively shallower waters. The relatively higher salinity and large amount of terrigenous material inputs are not conducive to the preservation of organic matters. In addition, the formation of dolomite minerals in saline lakes is controlled by both microbial activities and evaporation environment, and the origin plays an important role in sedimentary characteristics of lithofacies and accumulation of organic matters.