小型断陷咸化湖盆烃源岩发育特征与形成机制——以宁南盆地古近系清水营组为例

Developmental characteristics and formation mechanism of the source rock in a small faulted saline lacustrine: a case study of Palaeogene Qingshuiying formation in the Ningnan Basin

为了研究小型断陷咸化湖盆烃源岩发育特征与形成机制,通过对宁南盆地北部贺家口子剖面清水营组泥岩进行样品采集和有机地球化学测试与分析,研究烃源岩的岩石学组成和有机地化特征,评价其生烃潜力,探讨宁南盆地清水营组烃源岩发育特征和形成机理.结果表明,宁南盆地清水营组底部泥岩段生烃潜力相对较大,为低丰度烃源岩层位,有机质类型以Ⅱ型为主,处于低成熟生烃阶段.清水营组烃源岩以灰质泥岩和含膏泥岩为主,矿物组成上具有黏土矿物含量高且缺乏高岭石等特征;有机质为浮游生物和陆生植物的混合来源,以嗜盐浮游生物和细菌等为主,缺乏藻类生物;烃源岩发育水介质环境为高盐度-强还原和碱性环境.小型断陷咸化湖盆背景下的清水营组烃源岩发育在咸化湖盆演化早期的深水盐湖演化阶段,由于水体发生分层,表层水盐低,其中发育嗜盐的浮游生物、细菌等和间歇性河流带来的陆生植物组成了混合有机质来源;高盐度的底层水为静水的强还原环境,有利于沉积有机质的保存,是烃源岩形成的关键因素.

In order to study the development and formation mechanism of the hydrocarbon source rock in a small faulted saline lacustrine, the hydrocarbon potential of the mudstone, the development and formation mechanism of the hydrocarbon source rock of the Qingshuiying formation were analyzed via sample collection at the Hejiakouzi section as well as through an analysis of its organic geochemistry. The results showed the mudstone at the bottom of the Qingshuiying formation has a higher hydrocarbon potential,i. e. a hydrocarbon source rock with low abundance. The main organic matter type of the hydrocarbon source rock is type, being in the low maturity stage of hydrocarbon generation. The hydrocarbon source rock is mainly lime mudstone and gypsiferous mudstone, having the feature of abundant clay mineral and scarce kaolinite. The organic matter came from a mix of plankton and terrestrial plant, of mainly plankton and bacteria, lacking in algae. Aqueous medium for developing hydrocarbon source rock is high salinity,intense reducibility and alkaline environment with the feature of high hydrocarbon conversion, high hydrocarbon expulsion and early hydrocarbon generation. The hydrocarbon source rock in small faulted saline lacustrine developed in a deep saline lake at the early stage of the saline lacustrine. Mixed organic matter was formed by plankton and bacteria that developed under the delamination of water body and low surface salinity as well as terrestrial plant from intermittent river. The underlying water is lentic with a high salinity and intense reducibility, beneficial to the conservation of sedimentary organic matter and is the key factor for the formation of hydrocarbon source rock.