Characteristics and significance of heterogeneity of sea-land transitional facies shale gas reservoir in North Guizhou, China

Abstract

In order to identify the characteristics of the longitudinal heterogeneity of the sea–land transitional facies shale gas reservoir in the upper Yangtze region of North Guizhou, studies on the lithological combination, rock and mineral composition, geochemical parameters and reservoir microanisotropy characteristics of Longtan Formation in the study area are conducted on the basis of core observation, testing of geochemistry and reservoir physical property and well logging interpretation. The studies show that the lithological assemblages of the Longtan Formation are diverse and form an amina interbedding of “sand-mud-coal” with obvious cyclicity characteristics. There is a large longitudinal difference in rock and mineral composition and the average mass fraction of the clay mineral is 39.83%, which is obviously higher than that of the marine shale in North America and South China; the longitudinal heterogeneity of the organic matter abundance is high, with an average of 2.17% in the upper part, and 4.51% in the lower part; in accordance with the results observed with the scanning electron microscope and results calculated through pore fractal, the microscopic pore heterogeneity of the reservoir is high. The comparison and analysis of connecting wells with different scales in the study area show that the control effect of the depositional environment on longitudinal macroscopic heterogeneity of Longtan Formation is obvious, and the longitudinal microscopic heterogeneity is controlled through diagenesis. Meanwhile, studies with main coal mining seam as the seam section division method conclude that the heterogeneity of Coal Seam Sections 4 to 5 and Coal Seam Sections 13 to 15 is significantly smaller than that in other seam sections, and the Coal Seam Sections 4 to 5 and Coal Seam Sections 13 to 15 can be considered as a priority key seam section during development of shale gas.