Abstract
The pore structure and connectivity in petroleum reservoirs are controlled in part by their petrological properties. Mixed siliciclastic-carbonate rocks have complex compositions and heterogeneous spatial distributions of the various minerals. As a result, the study of the pore structure and connectivity of mixed siliciclastic-carbonate tight reservoirs has been limited. In this study, methods such as thin section microscopy, X-ray diffraction, X-ray computed tomography, low pressure N2 adsorption, and spontaneous imbibition were adopted to comprehensively analyze the petrological properties, pore structure, and connectivity of the mixed siliciclastic-carbonate tight reservoirs in the upper member of the Xiaganchaigou Formation in the Yingxi Area, Qaidam Basin. The results showed that micrometer-sized pores in mixed siliciclastic-carbonate tight reservoirs are mainly dissolution pores, and that the spatial distribution of the pores is highly heterogeneous. The average pore radius range, average throat radius range, and average coordination number range of micronmeter-sized pores are 2.09~3.42 μm, 1.32~2.19 μm, and 0.48~1.49, respectively. Restricted by the concentrated distribution of local anhydrite, the connectivity of micronmeter-sized pores develops well only in the anhydrite, showing negligible contribution to the overall reservoir connectivity. In contrast, nanometer-sized pores in the mixed siliciclastic-carbonate tight reservoirs are mainly intercrystalline pores in dolomite. The range of nanometer-sized pores diameters is mainly distributed in 1.73-31.47 nm. The pores have a smooth surface, simple structure, and relatively homogeneous spatial distribution. The dissolution of dolomite intercrystalline pores by acidic fluids increases the connectivity of the nanometer-sized pores. This paper presents genetic models for microscopic pore structures and connectivity of mixed siliciclastic-carbonate rocks, making possible the evaluation on the quality of the mixed siliciclastic-carbonate tight reservoirs.
Highlights
IntroductionMixed siliciclastic-carbonate deposits are defined as sediments consisting of both extrabasinal (e.g., epiclastic or terrigenous) and intrabasinal (autochthonous to parautochthonous) components, and they are an important type of continental sedimentary facies [1,2,3]
Mixed siliciclastic-carbonate deposits are defined as sediments consisting of both extrabasinal and intrabasinal components, and they are an important type of continental sedimentary facies [1,2,3]
The samples can be generally characterized by the presence of significant differences in the contents of various mineral types, as well as the significant discrepancies in the contents of the same mineral in different samples (Figure 2)
Summary
Mixed siliciclastic-carbonate deposits are defined as sediments consisting of both extrabasinal (e.g., epiclastic or terrigenous) and intrabasinal (autochthonous to parautochthonous) components, and they are an important type of continental sedimentary facies [1,2,3]. The sedimentary texture and mineral composition of mixed siliciclasticcarbonate tight reservoirs are by definition more varied than pure sandstones and carbonates [3,4,5]. Geofluids of the mixed siliciclastic-carbonate rocks [6,7,8,9,10,11,12,13]. Many breakthroughs have been made on the classification and depositional setting of mixed siliciclastic-carbonate rocks [5, 9, 10, 12]. Understanding the pore structure and connectivity of mixed siliciclastic-carbonate tight reservoirs, and the associated key controlling factors, is still lacking [14, 15]
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