Reservoir characterization of fluvio-deltaic sandstone packages in the framework of depositional environment and diagenesis, the south Caspian Sea basin

Highlights

• Fasila and Balakhany Suites as the main gas reservoirs in the South Caspian Basin.

• The drainage basin of the Volga River is the provenance for sandstone deposits.

• Primary sedimentary features and diagenesis both influence reservoir quality.

• Pore throats occluded by illite, smectite and chlorite with burial depth.

• Clastic facies analysis provides valuable insight to reservoir quality.

Abstract

The Middle Pliocene Fasila and Balakhany Suites are important petroleum-bearing reservoirs in the South Caspian Basin. There are four lithofacies associated with the interbedded sandprone reservoir intervals. The mineral constituents, sedimentary structures and depositional setting of the productive interval are interpreted as river-dominated depositional setting, including conglomeratic facies relating to a channel floor sub-environment and sandstone facies associated with the upper parts of river channels. The drainage basin of the Volga River is considered as the provenance for these deposits derived from basement rock outcrops to the north, which belongs to the Russian Precambrian semi-oceanic continental platform. Reservoir quality is assessed using the core-based porosity and permeability data in relation to the sedimentary and diagenetic processes influencing each lithofacies. Lithofacies 1 and 2 have good reservoir quality, while lithofacies 3 and 4 have poorer reservoir quality. Reservoir quality is controlled by both depositional setting (grain size and texture) and diagenetic imprints associated mainly with compaction, cementation and neumorphism. The role of clay minerals in the diagenesis of the sandstones with increasing burial depth is revealed by SEM and XRD analysis. Clay minerals progressively alter to illite, smectite and chlorite with increasing burial depth. This alteration leads to the occlusion of pore throats and degrading reservoir permeability.

Introduction

Siliciclastic rocks may contain significant information regarding their provenance and depositional environments, which can be assessed through studies of mineralogy, petrography and diagenesis (Kutterolf et al., 2008). Lithofacies analysis also plays an important role in the interpretation of depositional environments, reservoir quality and the assessment of formations as potential petroleum-producing zones. The results can be used in paleogeographic reconstructions, the study of paleo-climates and diagenetic processes (Suttner et al., 1981, Suttner and Dutta, 1986), as well as the subsidence history of sedimentary basins (Miall 2006; Catuneanu, 2006). In particular, sandstones continue to draw the attention of researchers due to their petrological diversity, wide range of sedimentary environments, tectonic histories and the frequency with which the incorporate zones displaying intrinsically high reservoir quality. The mineralogical composition of sandstones is typically controlled by the interplay of geological factors such as provenance, tectonic history and their post-depositional sedimentary processes (Dickinson, 1985).

Reservoir quality of clastic rocks is well known to be determined by lithofacies distribution, diagenesis and the nature and degree of fracturing. Component grain compositions substantially influence the mechanical and chemical stability of sand-prone packages (De Rosa et al., 1994). Moreover, grain sizes, sorting, roundness and matrix content largely determine the initial porosity of sandstones. The distributions of these sandstone specifications are in turn subjected to the sedimentary basin conditions that prevail post deposition, as they directly impact the type and severity of the diagenetic overprints. Clay mineral formation, part of the diagenetic process, typically results in a deterioration of sandstone reservoir quality. Clays are mainly formed as result of alteration (breakdown) of chemically unstable minerals and rock fragments into a suite of clay minerals, the combination of which depends on subsurface conditions and depositional mineralogy. It typically results in the formation of chlorite, smectite, zeolite and silica within the matrix and primary pore space (Utada, 2001). This area has been selected for study due to the presence of the large petroleum-bearing reservoir units of Pliocene age. The study has four distinct aims: 1) Petrographic and depositional setting analysis of Fasila and Balakhany Suites 2) Provenance analysis of the deep-water sandstone packages; 3) Studying the extent and significance of diagenetic processes; and, 4) The influence of facies and diagenetic processes on reservoir quality.