Study on Coal Forming Process and Coal Seam Distribution Controlled by the 4th-Order Sequence—A Case Study of T Block, the South Turgay Basin, Kazakhstan

Abstract

The South Turgay Basin is one of the important petroliferous basins in Kazakhstan. T Block is located in the south of the South Turgay Basin. Fault block traps in the lower Jurassic are extremely developed in the T block, and the potential of oil and gas resources is huge. The target layer in the study area is coal bearing sandstone reservoir, and it is difficult for the identification and prediction of the effective sandstone reservoir because of the development of the coal seam, so it is very important for the understanding of sandstone reservoir to study the formation process and distribution law of coal seam in T block of the South Turgay basin. Based on drilling data and seismic data, 4th-order sequences of the lower Jurassic J1ab formation have been divided. The lower Jurassic J1ab formation in this area develops 6 4th-order sequences. In the framework of the 4th-order stratigraphic framework, the formation environment, formation process and distribution law of the coal seam are analyzed. The study shows that the main coal forming environments in the study area are deltas, rivers, lakes and coastal environments. Taking delta as an example, the delta undergoes 6 stages to complete a complete coal accumulation process: stage S1–S2 is the channel sand deposition stage, stage S3–S4 is the concentrated sedimentary stage of coal seam, In the stage S5, the overlying water becomes deep and coal accumulation is stopped, during the stage S6, the channel retreated and the normal deltaic sedimentary system develops. Vertically, in the 4th-order sequence, the coal seam is undeveloped at the sequence boundary and at the maximum flood surface, while the coal seam is more developed in the ascending half cycle and the descending half cycle.