Utilizing borehole electrical images to interpret lithofacies of fan-delta: A case study of Lower Triassic Baikouquan Formation in Mahu Depression, Junggar Basin, China

Abstract

Large-scale conglomerate fan-delta aprons were typical deposits on the slope of Mahu Depression during the Early Triassic. Without outcrops, it is difficult to study the lithofacies only by examining the limited cores from the main oil-bearing interval of the Baikouquan Formation. Borehole electrical imaging log provides abundant high-resolution geologic information that is obtainable only from real rocks previously. Referring to the lithology and sedimentary structure of cores, a case study of fan-deltas in the Lower Triassic Baikouquan Formation of the Mahu Depression presents a methodology for interpreting the complicated lithofacies utilizing borehole electrical images. Eleven types of lithologies and five types of sedimentary structures are summarized in borehole electrical images. The sediments are fining upward from gravel to silt and clay in the Baikouquan Formation. Fine-pebbles and granules are the main deposits in T 1 b 1 and T 1 b 2 , but sandstones, siltstones and mudstones are more developed in T 1 b 3 . The main sedimentary textures are massive bed- dings, cross beddings and scour-and-fill structures. Parallel and horizontal beddings are more developed in T 1 b 3 relatively. On integrated analysis of the lithology and sedimentary structure, eight lithofacies from electrical images, referred to as image lithofacies, is established for the fandeltas. Granules to coarse-pebbles within massive beddings, granules to coarse-pebbles within cross and parallel beddings, siltstones within horizontal and massive beddings are the most developed lithofacies respectively in T 1 b 1 , T 1 b 2 and T 1 b 3 . It indicates a gradual rise of the lake level of Mahu depression during the Early Triassic, with the fan-delta aprons retrograding towards to the margin of the basin. Therefore, the borehole electrical imaging log compensate for the limitation of cores of the Baikouquan Formation, providing an effective new approach to interpret the lithofacies of fan-delta.

A grain size auto-classification of Baikouquan Formation, Mahu Depression, Junggar Basin, China

“…Benefitting from the ultrahigh-resolution, the grain sizes are clear to be classified in the visualized images. The spots within different gray values indicate the approximate gravelly grains: the bigger the spots, the coarser the gravels (Figures 4a, b, 5a, b and 6a, b) [24][25][26][27]. The sands transported by braided rivers or distributary channels of the fan-delta system are usually well sorted.…”

“…However, the coarse sandstones are bright (Figure 7a and b), medium sandstones are bright-yellow (Figure 8a and b) and fine sandstones are yellow (Figure 9a and b). It means that the coarser the sandy grains, the brighter the images [24][25][26][27].…”

“…The silts and clays accumulated in quiet water have well conductivity, and silty and muddy intervals have medium-low and the lowest resistivity, respectively. Correspondingly, electrical images of both are homogeneous brown (Figure 10a and b) and dark-brown colored, respectively (Figure 11a and b) [24][25][26][27].…”