渤海湾盆地渤中凹陷东营组烃源岩地球化学特征及生烃模式

对东营组烃源岩特征及其生烃机理认识不清,制约了渤海湾盆地渤中凹陷油气成藏认识与资源评价。利用有机岩石学、显微组分、孢粉鉴定及原油地球化学参数等资料,系统评价了东营组烃源岩及其生成原油的地球化学特征,探究了有机质生烃的影响因素及生烃模式。结果表明:东营组烃源岩主要为含黏土长英质泥岩和混合型泥岩,矿物组成以石英和黏土矿物为主,长石和碳酸盐矿物次之;东营组三段与东营组二段下亚段总有机碳(TOC)含量平均值分别为1.94%、0.87%,整体为好—优质烃源岩,显微组分以壳质组为主,有机质类型为Ⅱ₁、Ⅱ₂型,处于低成熟—成熟热演化阶段。东营组烃源岩具两期生烃特征,早期生烃高峰对应镜质体反射率(R_o)为0.6%,生成低成熟度烃类,晚期生烃高峰对应R_o为1.0%,生成成熟烃类。原油物性及生物标志化合物特征显示东营组以生成成熟油为主,含少量低熟油,原油整体具有低密度、低黏度、低含硫量、高含蜡量、高饱芳比、高Pr/Ph、高C₁₉/C₂₃三环萜烷值、低伽马蜡烷指数和低4-甲基甾烷指数等特征,属于混源有机质生源、弱氧化—弱还原沉积条件和淡水贫硫湖泊环境综合成因的轻质—中质原油。壳质组中的底栖藻无定形体及松科属孢粉等组分对低成熟—成熟阶段的烃源岩生烃具有重要贡献,黏土矿物和碳酸盐矿物及地层超压在一定程度上影响了烃源岩生烃演化过程。东营组烃源岩发育受淡水湖泊、特殊生物、无机矿物和超压等要素共同控制的双峰生烃模式。研究认识对于开展以渤中凹陷东营组为主力烃源灶的油气勘探具有重要参考意义。

Geochemical characteristics and hydrocarbon generation mode of source rocks of Dongying Formation in Bozhong sag,Bohai Bay Basin

The unclear understanding of the characteristics and hydrocarbon generation mechanism of source rocks in Dongying Formation has restricted the study on hydrocarbon accumulation and resource evaluation in Bozhong sag, Bohai Bay Basin. By use of organic petrology, macerals, sporopollen identification and geochemical parameters related to crude oil, this paper systematically evaluates the geochemical characteristics of source rocks and crude oil in Dongying Formation, and also investigates the influencing factors and modes of hydrocarbon generation from organic matters. The results show that source rocks in Dongying Formation are mainly clay-bearing felsic mudstone and mixed mudstone, of which the mineral composition is dominated by quartz and clay minerals, followed by feldspar and carbonate minerals; the average total organic carbon content (TOC) of Member 3 and the lower submember of Member 2 of Dongying Formation are 1.94 % and 0.87 % , respectively, indicating that the source rocks are generally of good and premium quality; macerals are mainly composed of exinite, and Type Ⅱ₁ and Ⅱ₂ organic matter are dominant, currently in the low-mature to mature thermal evolution stage. The source rocks of Dongying Formation is characterized by two-stage hydrocarbon generation: in the early stage, the hydrocarbon generation peak corresponds to a vitrinite reflectance (R_o) of 0.6 % , and the low-mature hydrocarbons are mostly produced; in the late stage, the hydrocarbon generation peak corresponds to a R_o of 1.0 % , and the mature hydrocarbons are mostly generated. The physical properties and biomarkers of crude oil indicate that crude oil in Dongying Formation is dominated by mature oil, supplemented by a small quantity of low-mature oil, which is characterized by low density, viscosity and sulfur content, high wax content, ratio of saturated hydrocarbon to aromatic hydrocarbon, Pr/Ph ratio and C₁₉/C₂₃ tricyclic terpane ratio, as well as low gammacerane index and low 4-methylsterane index, and belongs to the light-medium crude oil generated under the joint action of mixing organic matter, weak oxidation-weak reduction deposition conditions, and freshwater sulfur-poor lacustrine environment. The components such as amorphous benthic algae and Pinaceae sporopollen in exinite play an important role in hydrocarbon generation by source rocks in low-mature and mature stages; clay and carbonate minerals and formation overpressure have certain influence on the hydrocarbon generation evolution of source rocks. The development of source rocks of Dongying Formation is jointly controlled by the factors including freshwater lake, special organism, inorganic mineral and overpressure, which endows the source rocks with a bimodal hydrocarbon generation mode. This research can provide important reference for oil and gas exploration in the Dongying Formation of Bozhong sag as being the main hydrocarbon kitchen.