柴西北区新近系湖相细粒沉积岩油气富集条件

Hydrocarbon accumulation conditions of neogene lacustrine fine-grained sedimentary rocks in northwestern Qaidam Basin

柴西北区是柴达木盆地重要的油气资源接替区,新近系湖相细粒沉积岩沉积厚度大,有较好的非常规油气资源潜力。然而,由于咸化沉积环境、粒度小、岩石致密等条件的限制,细粒沉积岩成烃与成储机制复杂,油气富集条件尚不明确,勘探程度较低。以柴西北区新近系主力烃源岩上干柴沟组为研究对象,采集19口代表性钻井岩芯样品,分析样品有机质丰度、类型和成熟度,结合X衍射、岩石薄片、高压压汞、低温液氮吸附实验和扫描电镜,系统分析了其烃源岩特征、矿物组成、储集条件及影响因素,并优选出有利勘探区。研究表明:柴西北区上干柴沟组细粒沉积岩厚度普遍>500 m;烃源岩有机碳含量不高,有57%样品TOC含量<0.4%,20%样品TOC含量高于0.6%,少量样品TOC含量超过1.0%;有机质以Ⅱ1型和Ⅲ型为主,少量Ⅰ型有机质;样品Ro为0.54%~1.24%,处于低成熟—成熟阶段。样品非均质性强,孔隙结构复杂,介孔较发育,介孔孔隙体积增量为0.0010~0.0152 cm3/g,平均为0.0075 cm3/g。样品中氯仿沥青“A”含量与TOC含量、介孔体积均呈正相关性,与Tmax,BET比表面积及宏孔体积均无明显相关性。样品孔隙体积受到TOC含量、矿物含量及热成熟度控制。综合细粒沉积岩烃源岩及储集条件,初步优选干柴沟—油泉子—南翼山地区为柴西北区上干柴沟组致密油/页岩油勘探有利区。

Thick Neogene fine-grained sedimentary rocks were developed in the northwestern Qaidam Basin,where is an important replacement area for oil and gas resource exploration in the Qaidam Basin and has a tremendous unconventional oil and gas resource potential.However,limited by the salinized depositional environment,poor reservoir properties,and the largely unknown enrichment mechanisms of unconventional oil resource,the prospections on the fine-grained sedimentary rocks in northwestern Qaidam Basin are still situated at the initial stage.This research focused on the Shangganchaigou Formation of Neogene,that is a primary hydrocarbon source rock in the northwestern Qaidam Basin.Core samples from 19 representative drilling wells were collected to analyze the abundance,type,and maturity of organic matter.Based on X-ray diffraction,rock thin section,high-pressure mercury intrusion,low-temperature liquid nitrogen adsorption,and SEM(scanning electron microscopy)data,the hydrocarbon source rock characteristics,mineral composition,accumulation conditions and their controlling factors in the fine-grained rocks of the Shangganchagou Formation was systematically investigated.Moreover,this research also identified the favorable horizons for hydrocarbon exploration.The result shows the thickness of the fine-grained rocks of the Shangganchaigou Formation in the study area is generally more than 500 m.Relatively low TOC contents present in the all studied samples,with 57%samples are less than 0.4%,20%samples are more than 0.6%,and a few more than 1.0%.Organic matter in these rocks samples is mainly typeⅡ1,Ⅲand few typeⅠ.The vitrinite reflectance(Ro)is ranging from 0.54%-1.24%exhibiting thermally low mature to mature stage.Pores with complex structure and strong heterogeneity are developed in the samples.The incremental pore volume of mesopores is 0.0010-0.0152 cm3/g,and the mean mesopore volume is 0.0075 cm3/g.The chloroform bitumen“A”content of the samples has a positive relationship with TOC content and mesopore volume,while it has no obvious correlation with Tmax,BET surface area,and macropore volume.The pore volume of the samples is controlled by TOC content,mineral composition,and the thermal maturity.Based on the comprehensive evaluation of hydrocarbon generation and accumulation conditions of the Shanggaichaigou fine-grained rocks,it is found that there is great tight oil/shale oil potential in the Ganchaigou-Youquanzi-Nanyishan area in the northwestern Qaidam Basin.