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

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 type Ⅱ1 ,Ⅲ and few type Ⅰ. 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 volumeis 0. 0075cm3/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.