Structural and sedimentological controls on diagenesis in the Ravenspurn North gas reservoir, UK Southern North Sea

Abstract

The Rotliegend Lower Leman Sandstone Formation has been studied near the northern limit of its development in Ravenspurn North to determine the influence of structure, sedimentology and diagenesis on reservoir quality. The Lower Leman Sandstone in this area is about 300 ft thick and comprises six depositional cycles which reflect alternating periods of relatively dry and wet climate. Aeolian sands form the best reservoir and formed as small westward-migrating transverse dune fields and draa, at most 60 ft thick. These dunes were frequently reworked by southerly derived, strong fluvial sheetfloods which drained into the Silverpit playa lake. The Ravenspurn area is divided into main northwest-southeast-trending fault blocks which are markedly different in terms of their diagenetic evolution and reservoir performance. The northeasterly B structure contained gas earlier and was unaffected by mid- to late Jurassic illitization. The southwesterly A structure was uplifted later and received accumulated gas after reservoir quality was reduced by pervasive illitization. A hierarchy of factors influenced the diagenesis. These were the primary lithological variation, burial history and the structural development of the Ravenspurn North area. Depositional processes also influenced diagenesis; the deposition of allogenic clay and the formation of early quartz, non-ferroan dolomite and anhydrite reduced the reservoir quality of fluvial sheetfloods. Burial diagenesis resulted initially in ferroan dolomite, kaolinite and later quartz precipitation in available primary and secondary porosity. Stable isotope and fluid inclusion studies indicate that ferroan dolomite and later quartz precipitated at about 100°C in Triassic-early Jurassic times from reduced fluids derived partly from the Carboniferous basement. Mixing of these fluids with more alkaline connate water was the main triggering mechanism for authigenic mineral formation. Gas accumulation took place first in the northeasterly B structure which had early closure; the last authigenic phase to precipitate here was poikilotopic siderite at temperatures of about 120°C. Elsewhere diagenetic fluids evolved to a more alkaline state and widespread illitization took place which particularly affected more permeable aeolian facies. The illitization reduced the reservoir quality of the Lower Leman Sandstone and contributed to diagenetic sealing (to the northwest) of the field. K–Ar dating indicates that peak illitization took place between 150–170 Ma (mid–late Jurassic). Subsequent periods of uplift in the late Cimmerian and particularly during the early Tertiary–Miocene produced the final structure of Ravenspurn North and the spillage of gas into this structure. The combination of structural and diagenetic events explains the differences in reservoir quality and well performance of the two structural blocks in the field.