Diagenetic controls on the reservoir quality of a Lower Carboniferous Tight Gas Sandstone: The Breagh Field.

Abstract

The Breagh Field is the first and presently the only field developed within the Lower Carboniferous clastic reservoir sequence in the UK SNS. Breagh is estimated to contain 909 bcf (P50) in tight (low permeability) sandstones with an expected recovery factor of 50%. The reservoir quality has been strongly influenced by diagenetic processes which significantly reduced porosity and permeability, and increased reservoir heterogeneity. This study integrates quantitative petrographic data, cathodoluminescence study, stable isotope analyses and 3D-XCT to investigate the impact of diagenesis and depositional facies variation on reservoir quality of the Breagh Sandstone by investigating the clay-poor sandstones within the reservoir.

Based on examination of core log and petrographic analyses, eleven facies associations were identified. The primary reservoirs are the stacked braided fluvial channel deposits although numerous thinner and finer-grained sandstone bodies exist within the delta front sands and abandoned channel facies. The connectivity of these thinner, secondary reservoirs will be critical for adding pay at Breagh and the other potential gas fields identified to the SSW, NE, and West of the Breagh Field.

The Breagh Sandstone consists of very fine- to coarse-grained, moderately well sorted to very well sorted, arenite to sublitharenite sands. The reservoir properties of the sandstone are relatively poor and with permeabilities between 0.1-100mD and porosities in the range of 9.5–19.6%. COPL and CEPL analysis show that mechanical compaction is the dominant process for the destruction of pore spaces, leading to a porosity reduction of 22.6% to 27.68% but, cementation still accounts for further reduction of porosity to the current range of 14.8% to 17.4%.

Features observed during early diagenesis include alteration of iron bearing silicates, clay coats, early carbonate cements, early quartz overgrowth, precipitation of sulphates and dissolution of feldspars. Mid-stage diagenetic events include dissolution of earlier formed sulphates, formation of kaolinite and illitization of mica. Late diagenetic features include late carbonate cementation, clay mineral cementation (illite and late-stage kaolinite), precipitation of further iron oxides, dissolution of illite and carbonate cements. Barite and anhydrite are minor late diagenetic cement phases.

Further investigation using SEM 2D images and 3D-XCT show that permeability is primarily controlled by the distribution of pore types, pore throat radius and pore connectivity. Whilst inter-grain pores (larger than 22μm) have the most potential for reservoir flow, the large number of smaller pore-throats do have negligible contribution to reservoir flow potential, albeit more significant for the reservoir storage capability.

The diagenetic control on the studied sandstone as well as the extracted pore network here will be a good input in predicting multi-phase flow properties of the studied reservoir and other potential gas fields. In particular, understanding the pore network and distribution of the thinner, secondary reservoirs will be critical for increasing producibility at the Breagh Field itself.