Stratigraphic evolution of Stage-3 clinoforms from northern Tarim Basin: Evaluating controlling factors on a distal steepening carbonate ramp growth and shallow-water sedimentation

Prograding clinoforms of different scales usually occur in special carbonate sedimentary systems. Since carbonates are mainly generated in situ by biochemical processes within the basin, prograding clinoform sediments are less common than terrigenous clastic sedimentary systems. Carbonate prograding clinoforms have not received much attention in research, resulting in a lack of investigation into their sedimentary process, genetic mechanism and controlling factors. The investigation utilized integrated analysis of cores, thin sections, 3-D seismic and well logging data to explain the sedimentary process, genetic mechanism, controlling factors of carbonate clinoforms and its relationship with the development of favorable reservoirs. The result shows that there are five vertically overlapped clinoforms advancing from WWS to NNE. which occurs as S-shaped lens, with thick middle parts and thin ends. Well logging and seismic characteristics have provided the basis for establishing a sequence-stratigraphic framework. Two regionally third-order sequences (Sq1-Sq2) are developed in the Stage-3 succession, Sq1 incorporates three clinoforms (C1-C3); Sq2 incorporates two clinoforms (C4-C5). In the Sq1, the vertically aggradational thickness of the clinoforms were large, about 70~160 m, and the study area was dominated by the outer middle-ramp deposits. In the Sq2, the vertically aggradational thickness of the clinoforms were small, about 3~40 m, and shallow shoal deposits were developed in the edge of the clinoforms. The clinoforms can be further divided into 21 2^rd-clinoforms. Four main types of clinoform architecture were recognized: falling (progradational pattern), flat (progradational pattern), rising (progradational and aggradational pattern) and backstepping (retrogradational pattern) edge trajectories. The present study revealed that the formation of clinoforms were controlled by the change of eustasy. Due to the broad ramp sedimentary background and sea-level fall, the sedimentary accommodation space continued to decrease, which ultimately lead to the formation of the prograding clinoforms. Reservoirs dominated by high-energy shoal are developed at the edge of the clinoforms.