Depositional sequences and microfacies of Lower Carboniferous strata in the Marsel block of the Chu-Sarysu Basin, Southern Kazakhstan

Abstract

The sequence development and evolution of Lower Carboniferous strata in the Marsel block of the Chu-Sarysu Basin, Kazakhstan have been established through detailed sedimentological analysis of cores, petrophysical data, and new seismic interpretations. Four depositional sequences and eighteen parasequences were classified in the Lower Carboniferous strata. Each depositional sequence is composed of a transgressive systems tract (TST) and highstand systems tract (HST), roughly coinciding, respectively, with the following four stratigraphic intervals (1) lower Tournaisian (C1t) through lower Visean (C1v-1) strata; (2) middle Visean (C1v-2) strata; (3) upper Visean (C1v-3) strata; and (4) Serpukhovian (C1sr) strata. Parasequences are bounded by an instantaneous drowned punctuated surface (C1t and C1v) or instantaneous exposed punctuated surface (C1sr). In addition, petrographic studies have led to the recognition of 20 microfacies, distributed on the major depositional facies zones of the outer ramp (Mf1, Mf6, Mf20), middle ramp (Mf3–5, Mf8–13), and inner ramp (Mf2, Mf4, Mf7, Mf9, Mf12, Mf14–19). They are grouped into seven facies associations corresponding to widespread deposits of tidal flats (MA3, MA5), shallow shoals (MA6), and shallow lagoons (MA4) in the inner ramp; deposits of restricted lagoons (MA1) and local patch shoals (MA7) in the middle ramp and a relatively small outer ramp deposits (MA2). SQSQ All the microfacies and their associations are grouped into the following 6 major facies belts: outer ramp; restricted lagoon, and patch shoals in the middle ramp; and shallow shoals, restricted lagoon, subtidal to intertidal and supratidal in the inner ramp. The interpretation of these facies belts suggests a shallow warm-water tropical-subtropical depositional environment on a passive continental margin ramp. Facies correlation between wells in sequence framework suggests that the strata may be divided into four sequences (SQ1, SQ2, SQ3, SQ4). SQ1 and SQ2 are dominated by lagoon facies and outer ramp facies in the northwest area in the TST, and tidal flat facies in the HST; SQ3 is characterized by restricted to semi-restricted lagoon facies in the TST and local patch shoal facies and mixed tidal flat facies in the HST; SQ4 is marked by restricted lagoon facies in the TST and evaporitic lagoon, tidal flat, and shallow shoal facies in the HST. The research results can be used for investigating the development and distribution of potential reservoirs in the Lower Carboniferous strata of the Chu-Sarysu Basin; the reservoirs in the HST of late Visean and Serpukhovian in the northern and southeastern part of the platform interiors may be of premium quality.