Effect of lithofacies on differential movable fluid behaviors of saline lacustrine fine-grained mixed sedimentary sequences in the Jimusar sag, Junggar Basin, NW China: Forcing mechanisms and multi-scale models

Abstract

The crude oil movability, acting as one of the controlling parameters for estimating how difficult it is to exploit petroleum resources from lacustrine shale reservoirs, had still not been investigated systematically, especially for the saline lacustrine fine-grained mixed sedimentary series. How to establish an effective assessment criterion which is suitable for deciphering shale oil movability classically, and how to probe the root cause of the differential movable fluid saturation in heterogeneous lithofacies ? In order to unravel the causal linkage and confirm a coupling relationship between lithofacies heterogeneity and hydrocarbon fluid flow activity for the saline lacustrine fine-grained mixed sedimentary series, we present a set of new results from organic geochemistry analysis, FE-SEM imaging, low-pressure N2 adsorption, high-pressure mercury injection intrusion experiments (MIP), and nuclear magnetic resonance (NMR) experiments from the targeted dessert reservoir of the saline lacustrine fine-grained mixed depositional systems of the Middle Permian Lucaogou Formation in the Jimusar Sag, Southern Junggar Basin, NW China. Our results demonstrated a relatively obvious triple-level distribution of the movable fluid-bearing capacity and also reservoir quality for the dominant five identified lithofacies. The sandy dolomitic sandstones and felsic siltstones act as the first-class categories mainly characterized by a highest total pore volume range and also pore volume within pore size range of 50–300 nm, as well as a relatively lowest D2 values defined as the pore-throat fractal dimension of movable fluid. Secondly, both the dolarenites and dolomitic mudstones are considered as the medium quality reservoirs, and particularly noteworthy is a development of the characteristic microfractures in dolomitic mudstones greatly facilitates the formation and widespread distribution of pore-fracture network systems, leading to a relatively better storage capacity for movable fluid as suggested by relatively higher pore volume and lower fractal dimension values compared with those of micritic dolomites suspected as the most unfavorable category. More importantly, a multi-scale detailed model is proposed to illustrate the potential linkage spanning a total of three different scales respectively as lithofacies associations, microscopic reservoir textures and architectures, as well as fluid occurrence behaviors. This work can be considered as an analogue to better understand the spatial distribution patterns of dessert reservoir intervals and differential recoverability of saline lacustrine mixed-source shale oil resources, and to provide further conceptual basis for optimization of development engineering technique schemes.