Utilizing borehole electrical images to evaluate sorting of conglomeratic formation: example from Lower Triassic Baikouquan Formation in Well M152, Mahu Depression, Junggar Basin, China

Abstract

Grain-size sorting of buried formation in openholes only comes from the particle size analysis of drilled cores. However, cores impossibly cover the whole formation, and the sorting of uncored interval is absent. With ultrahigh-resolution, borehole electrical image contains particular geological attributes, such as grain-sizes, sedimentary structures, and facies. In this paper, a sorting evaluation method is proposed based on conductivity spectrum of borehole electrical image from Baikouquan Formation in Well M152, Mahu Depression. The first stage is revising the depth of the cores by calibrating the cores to the borehole electrical images. The second stage is computing the two-dimensional conductivity spectrum of conductivity image. The third stage is calculating the sorting index from conductivity spectrum. The fourth stage is converting the sorting index to the evaluated sorting by a mathematical equation between sorting index and laboratorial grain-size sorting. In the cored interval of the Well M152, the evaluated sorting is compared with the grain-size of gravels; what can be concluded is that the evaluated sorting values from borehole electrical images gradually reduce with the decrease of the gravelly grain-sizes. The proposed method is demonstrated be beneficial to reflect the vertical diameters and sorting of gravelly grains in deeply buried conglomeratic formation continuously.