Comparison of diagenesis and reservoir quality of microporous lime mudstones (Aptian) between anticline crest and flanks: Abu Dhabi, United Arab Emirates

Abstract

The impact of in-situ and migrated oil on diagenesis and reservoir quality of microporous (porosity ≤ nil to 26%; permeability ≤ nil to 3 mD) intra-shelf basinal lime mudstones (Aptian) is constrained by comparing samples from anticline crest (i.e. oil zone) versus flanks (i.e. water zone), Abu Dhabi, United Arab Emirates. Higher porosity and permeability and larger pore-throat radii in the crest than in the flank mudstones is attributed to retardation of calcite cementation in the crest due to oil emplacement. Calcite cement has precipitated as syntaxial micro-overgrowths around micrite particles and as equant microspar in moldic pores and small vugs. Deviation of a large number of samples from this crest versus flanks porosity and permeability trend reflects the impact of other parameters, including: (i) subtle grain-size variations (ii) proportion of intact (i.e. with intragranular pores), versus broken coccolith tests, and (ii) degree of calcite cementation. Calcite cement was sourced by stylolitization of the limestones, which is more frequent and extensive in the flanks than in the crest. Similar bulk δ18OVPDB values of calcite in the crest (−7.3‰ to −4.6‰) and flanks (−6.9‰ to −5.5‰) is attributed to closed diagenetic system in the microporous mudstones, which masks the role of burial temperature in fractionation of oxygen isotopes during continued cementation of the water-saturated flank mudstones. The presence of miscellaneous saddle dolomite in the crest and flanks is attributed to the field-wide flow of hot basinal fluids along fractures and stylolites. A conceptual model unravelling the diagenetic and related reservoir-quality evolution pathways of mudstones across an oilfield anticline has been developed. This model has important implications for exploration of tight hydrocarbon reservoirs and improves our understanding of oil migration and emplacement in source rocks.