Facies, Architecture and Diagenesis of Middle to Upper Jurassic Carbonates: An Outcrop Analogue for Subsurface Reservoir Prediction (Ghissar - Uzbekistan)

Abstract

Abstract

Recent investigations have been carried out in the Ghissar range (Uzbekistan) to study the development and the architecture of a vast Middle-Upper Jurassic carbonate platform. The Ghissar Mountains consist of a succession of anticlines and synclines forming a relief between the Amu Darya and Afghan-Tajik basins. The studied series crops out as continuous limestone bars overlying Lower to Middle Jurassic continental then marine siliciclastic deposits. Its thickness ranges from 130 m in the northern Ghissar to 500 m in the central and southern Ghissar. The main objectives of this study are (1) to establish a high resolution stratigraphic framework for these deposits and (2) to determine the diagenetic history of the carbonates to (3) help in the prediction of subsurface reservoirs.

The reconstruction of the deposition profile indicates that during Middle to Upper Jurassic, the Ghissar was a gently dipping ramp. Two major sequences have been recognized, one being Callovian in age and the other potentially Oxfordian to Lower Tithonian. The first sequence is subdivided into seven fourth-order transgressive-regressive sequences. In the Callovian sequence, oobioclastic grainstones were deposited in the permanently agitated shallow water zone, while patch/pinnacle reefs build up distally, close to the storm-wave base. Small shallow water deltas, supplied in detrital particles by the erosion of the Hercynian relief, developed in the northern margin of the Ghissar. The second large-scale sequence is composed of peritidal deposits, showing rapid vertical and lateral transitions, drawing a more complex architecture. The series consists in meter-thick alternations of ooid grainstones, pellets/algal pack- to grainstone and gypsum pseudomorphs mudstone. Gypsum content increases toward the top of the platform, indicating the development of hypersaline environments during the restriction of the Amu Darya basin (Late Oxfordian? to Tithonian?).

The studied carbonates present complex mineralogical parageneses formed by fluid circulation during the burial and the exhumation of the Ghissar. The dolomitization and secondary dissolution of burrows is one of the diagenetic pathways allowing to create and preserve porosity in these carbonates. Moreover, mineralogical evidence of Thermal Sulfate Reduction (native sulfur in reef cavities; pyrite associated with bitumen), have been found in the Ghissar. The role of TSR on dolomitization and reservoir properties needs to be evaluated.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016