Advantageous source–reservoir combination in siliciclastic–carbonate sedimentary systems in a saline lacustrine basin (Jiuquan Basin, NW China): Implications for prediction of tight oil enrichment

Abstract

Unconventional tight oil reservoirs have recently emerged as a significant source of oil worldwide, and juxtaposed to organic‐rich source rocks, tight mixed siliciclastic–carbonate sedimentary reservoirs are widely found. In China, such reservoirs are mainly found in saline lacustrine basins. In this paper, we conduct a systematic study on advantageous source–reservoir combinations (SRCs) in the Cretaceous Xiagou Formation in the Qingxi Sag of the Jiuquan Basin, a tight saline lacustrine and mixed siliciclastic–carbonate sedimentary oil reservoir. Mixed sediments result in multiple SRCs as follows: self‐contained SRC, thick interbedded‐type SRC, thin interbedded‐type SRC, and down generated‐up stored SRC. Geochemical characteristics of organic matter in the Cretaceous Xiagou Formation are established on the basis of Rock‐Eval pyrolysis and total organic carbon (TOC) and vitrinite reflectance analyses. TOC (0.5–4.12%, av. 1.62%) and S2 (0.36–26.63 mg/g, av. 5.62 mg/g) indicate a good hydrocarbon generative potential. The organic matters contain predominantly Type II kerogen with minor contributions from Type I kerogen. The samples analysed have vitrinite reflectance in the range of 0.65–1.0%Ro, and pyrolysis Tmax in the range of 430–450 °C indicate that the source rocks are early mature to mature and within the main stage of oil generation. The tight oil resources are divided into 4 classifications according to TOC versus S1 diagram, and dolomitic mudstone in lower K1g3 and K1g1 is primarily considered to be potentially generative and productive. Reservoir characterization such as porosity, permeability, and pore‐size distribution derived from nuclear magnetic resonance indicates that nano‐meter‐sized pores dominate pore size distribution, regardless of the lithology. Fractures contribute to the creation of relatively higher productivity reservoirs. The statistics of oil testing results shows that the daily oil productions increased basically from the down generated‐up stored SRCs to thin interbedded‐type SRCs to thick interbedded‐type SRCs to self‐contained SRCs. The movable oil saturations derived from nuclear magnetic resonance have the same changes as daily oil production. The self‐contained SRCs and the thick interbedded‐type SRCs are identified as advantageous SRCs that are important for tight oil prediction.