The Accumulation Model Of Tight Oil In Lower Part Of The 3^rd Member Shahejie Formation Of The Shulu Sag,jizhong Depression

The Shulu sag in the Jizhong depression is a key area of continental tight oil exploration in China.The lower Es₃ in the study area are potential prospect for tight oil accumulation.The main conclusions are as follows:The lower Es₃ organic matter-rich marlstone in the Shulu sag are developed in this area with the maximum thickness up to 200m.Continuous thickness of marlstone is bigger than 20m in every single individual third-order sequences.the TOC of marlstone mostly ranges between 0.6%-3.0%.The major type of the organic matter in the study area is type II1,partly including type I somewhere.Marlstone also has moderate thermal maturity mostly ranging from 0.5%to 1.8%(vitrinite reflectance Ro),basically in the oil-wet gas window.The main minerals composition in the study include calcite,dolomite,clay minerals,quartz,feldspar and pyrite.Among them,calcite is the predominant mineral.The average content of carbonate minerals in the study area is 66.5%,that of clay mineral is 23.8%,and the rest minerals content sum is 9.7%.The reservoir have poor porosity and permeability.The conglomerate average porosity at 1.6%and permeability at 1.37m D;The marlstone porosity average at 1.1%and permeability average at 1.12 m D.This tight reservoir mainly develops micron-and nano-scale pores.The most important reservoir space types include organic matter pores,intergranular pores,intercrystalline pores,dissolution pores,etc.A hydrous pyrolysis experiments were conducted to simulate the process of hydrocarbon generation in the source rock with the organic-rich thermally immature samples in the Shulu Sag,the result shows that the total amount of generated hydrocarbons is 450 mg HC/g origin TOC.The amount of retained hydrocarbon(tight oil)is controlled by dual control of the amount of hydrocarbon generation and hydrocarbon expulsion:The amount of tight oil resources increase with increasing maturity which is controlled by hydrocarbon generation in the early oil window,decrease with increasing maturity which is controlled by hydrocarbon expulsion in the late oil window.Two different types of tight oil accumulation models are developed in the Shulu sag.The marlstone one is controlled by the TOC content and thermal maturity of source rock.The degree of fracture development and the connectivity of high quality source rocks mostly controls the conglomerates ones oil accumulation.Analyses of the tight oil accumulation in the Shulu Sag have shown that they are distributed unevenly both horizontally and vertically.In the plane,it can be divided into three regions.Vertically,the Shulu sag includes two system.The third-order sequence 1 and2 are fracture communication type accumulations,whereas the third-order sequence 3,4 and 5 tight oil are self-generating and self-preserving.In this paper,we propose a model of hydrocarbon generation and expulsion in the continental tight carbonate system,reveal the main controlling factors of tight oil enrichment,and establish a accumulating model for tight oil in the Shulu sag.This paper is helpful to better understand the tight oil accumulation mechanism,and enable to better evaluate and predict for tight oil distribution in the Shulu sag and other basins with similar geological condition.