DIAGENETIC CONTROLS ON THE RESERVOIR QUALITY OF THE TIGHT GAS COLLYHURST SANDSTONE FORMATION, LOWER PERMIAN, EAST IRISH SEA BASIN, UNITED KINGDOM

Abstract

Research on tight gas sandstones and other unconventional reservoirs has been on the increase due to significant hydrocarbon reserves within them which contribute to the global economy. Diagenetic processes influence tight gas sandstone reservoirs, reducing reservoir quality. This study employs petrographic, mineralogical and stable isotopic analyses on two different onshore core successions and outcrop data to investigate the composition and texture, the diagenetic history, and the impact of lithology and diagenesis on reservoir quality and gas composition of the tight-gas Collyhurst Sandstone Formation (CSF) in the East Irish Sea Basin of north-western UK. Advanced X-ray Computed Tomography (XCT) technique was further applied to characterise the 3D pore structure of the sandstone. The CSF consists of fine- to very coarse-grained, poorly to well-sorted, sub-arkosic to lithic arenite sandstones. The reservoir properties of the sandstone are relatively poor, with average porosity and permeability of 6.6% and 0.387 mD respectively. Five lithofacies were identified on the basis of lithological characteristics, sedimentary structure and microscopic observation; conglomerate with sandy matrix (Cs), horizontally laminated sandstone (Hs), planar cross-laminated sandstone (Pc), laminated fine-grained sandstone (Fs) and laminated mudstone lithofacies (Lm). Contrasting characteristics between the reservoir units suggest the development of divergent diagenetic pathways. Major diagenetic cements during early diagenesis include compaction, growth of pore-filling early carbonate cements (calcite I, dolomite and siderite I) and vermicular kaolinite. Late diagenetic features include further carbonate cementations (ferroan dolomite/ankerite, calcite II and siderite II-III), clay mineral cementation (illite and chlorite) with both grain-coating and pore-filling textures. The first order control on the studied reservoir is the primary composition governed by provenance (textural and mineralogical composition) and the depositional environment. The effect of diagenesis is observed in compaction processes, accounting for up to 48.5% initial porosity loss of the sandstone, and cementation which contributes to about 32% of porosity loss. Chemical reactions during burial diagenesis are also interpreted to have influenced gas composition of the reservoir. The combination of petrographic and geochemical analyses assists in interpreting the diagenetic evolution of the studied sandstones, which was valuable in reconstructing the paragenetic sequence. Further analysis with XCT provides a useful tool for imaging and quantification of microstructure of the sandstone allowing identification of pore types, geometry, distribution and connectivity which may influence petroleum production. The overall findings from this study provide conditioning information useful in predicting reservoir quality and may inform and guide future exploration and appraisal, as well as development and production in similar tight-gas sandstone settings where the distribution of authigenic minerals, chemical reactions, and migration pathways remain unclear.

Date of Award	1 Aug 2019
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Kevin Taylor (Supervisor) & Michael Bowman (Supervisor)