TURNER, PETER, Birmingham University, Birmingham, United Kingdom; MICHAEL JONES, Hamilton Oil Co. Ltd., London, United Kingdom; JEREMY PROSSER, Aberdeen University, Aberdeen, Scotland; and GRAHAM WILLIAMS, Keele University, Keele, United Kingdom

ABSTRACT: Structural and Sedimentological Controls on Diagenesis in the Ravenspurn North Gas Reservoir United Kingdom Southern North Sea

The Ravenspurn area is divided into two 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 Middle to Late Jurassic illitization. The southwesterly A structure was uplifted later and received accumulated gas after reservoir quality was reduced by pervasive illitization.

The deposition of allogenic clay and the formation of early quartz, nonferroan dolomite, and anhydrite reduced the reservoir quality of fluvial sheetflood. 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 degrees C in the Triassic-Early Jurassic from reduced fluids derived partly from the Carboniferous basement. Gas accumulation took place first in the northeasterly B structure, which had early closure. Elsewhere diagenetic fluids evolved to a more alkaline state, and widespread illitization took place which particularly affected more permeable eolian 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 and 170 Ma (Middle-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.

AAPG Search and Discovery Article #90990©1993 AAPG International Conference and Exhibition, The Hague, Netherlands, October 17-20, 1993.