Abstract
The Upper Jurassic, shallow marine Corallian sandstones of the Weald Basin, UK, are significant onshore reservoirs due to their future potential for carbon capture and storage (CCS) and hydrogen storage. These reservoir rocks, buried to no deeper than 1700 m before uplift to 850 to 900 m at the present time, also provide an opportunity to study the pivotal role of shallow marine sandstone eodiagenesis. With little evidence of compaction, these rocks show low to moderate porosity for their relatively shallow burial depths. Their porosity ranges from 0.8 to 30% with an average of 12.6% and permeability range from 0.01 to 887 mD with an average of 31 mD. The Corallian sandstones of the Weald Basin are relatively poorly studied; consequently, there is a paucity of data on their reservoir quality which limits any ability to predict porosity and permeability away from wells. This study presents a potential first in the examination of diagenetic controls of reservoir quality of the Corallian sandstones, of the Weald Basin’s Palmers Wood and Bletchingley oil fields, using a combination of core analysis, sedimentary core logs, petrography, wireline analysis, SEM-EDS analysis and geochemical analysis to understand the extent of diagenetic evolution of the sandstones and its effects on reservoir quality. The analyses show a dominant quartz arenite lithology with minor feldspars, bioclasts, Fe-ooids and extra-basinal lithic grains. We conclude that little compactional porosity-loss occurred with cementation being the main process that caused porosity-loss. Early calcite cement, from neomorphism of contemporaneously deposited bioclasts, represents the majority of the early cement, which subsequently prevented mechanical compaction. Calcite cement is also interpreted to have formed during burial from decarboxylation-derived CO2 during source rock maturation. Other cements include the Fe-clay berthierine, apatite, pyrite, dolomite, siderite, quartz, illite and kaolinite. Reservoir quality in the Corallian sandstones show no significant depositional textural controls; it was reduced by dominant calcite cementation, locally preserved by berthierine grain coats that inhibited quartz cement and enhanced by detrital grain dissolution as well as cement dissolution. Reservoir quality in the Corallian sandstones can therefore be predicted by considering abundance of calcite cement from bioclasts, organically derived CO2 and Fe-clay coats.
Highlights
Diagenesis involves physical, chemical and biological processes which act on texturally and compositionally unstable sediments, causing them to reach mineralogical and textural maturity and converting sediment into rock [1]
High resolution sedimentary core logging of three wells: Bletchingley 5 (BL5), Palmers Wood 3 (PW3) and Palmers Wood 7 (PW7), was carried out at the British Geological Survey (BGS) core store in Keyworth, Nottinghamshire
About 17 m of core was available from PW3, 17 m of core was available from PW7 and nearly 20 m of core was available from BL5
Summary
Diagenesis involves physical, chemical and biological processes which act on texturally and compositionally unstable sediments, causing them to reach mineralogical and textural maturity and converting sediment into rock [1]. Diagenetic processes alter sedimentary rock’s porosity and permeability (reservoir quality) as burial progresses [2]. Understanding the controls and processes involved in diagenetic alteration is useful in predicting reservoir quality evolution [3]. The specific characteristics of primary depositional systems are paramount for reservoir quality in shallow reservoirs but they become relatively less important as burial proceeds and as mesogenetic processes become dominant [4]. For shallowburied, diagenetically-simple sandstones, understanding the dominant depositional controls (texture and composition) may be sufficient to predict reservoir quality [4]. Understanding the controls on eogenetic processes can permit prediction of reservoir quality in rocks that have not entered the mesogenetic realm. This study of iron-rich and bioclastic Jurassic sandstones at a relatively shallow present-day depth of about 850 to 900 m (true vertical depth, TVD) in the Weald Basin, UK, sheds light on eogenetic processes
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