Improving Geological Models for Redevelopment of Old Fields: Comparison of Natih Formation Outcrop and Subsurface Data, Northern Oman

Peter Homewood¹, Quintin Davies², Volker Vahrenkamp³, Xavier Janson⁴, Carine Grelaud⁵, Henk Droste⁶, Philippe Razin⁷, Omar Al Ja'Aidi⁵, Deborah Bliefnick², and Alia Al Bahry^3
1 Sultan Qaboos University, Al Khod, Oman
² Badley Ashton and Associates, Lincolnshire LN9 6PB, England
³ Petroleum Development Oman, Muscat, Oman
⁴ Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, Austin
⁵ Sultan Qaboos University Carbonate Centre,
⁶ JVR Centre Carbonate Research, Muscat, Oman
⁷ Bordeaux University, Bordeaux, France

Industrial studies are being carried out by Petroleum Development Oman to revise 3D models of the Albian to Turonian Natih Formation, a prolific reservoir unit. Facies and facies associations have been determined both on cores, and on analogous outcrops, allowing a close comparison. Many facies are common to both the field and to the outcrops, whereas some are shown to be restricted to specific palaeogeographic settings (e.g. intrashelf basin centre or margin, structural highs, fault zones). Better vertical continuity of un-weathered sequences documented from cores was complemented by observations on lateral continuity of facies and facies substitutions from the outcrops. Meter to kilometre scale stratal geometries and stratigraphic surfaces were observed on outcrops and recorded with LIDAR imaging. Well logs were correlated at the regional scale with several successive datum lines, taking outcrop and subsurface observations (including seismic) into account. Whereas a single facies model was used for previous static reservoir models, seven different facies models are now constructed for the Natih, depending on the change in depositional profile (reconstructed from facies associations and seismic geometries). Separate models were constructed for transgressive and regressive modes, since a number of facies prove to be restricted to one or the other. The distributions of facies and facies associations vary considerably between the seven scenarios. Differences are governed by the evolution of deposition under changing relative sea level, and the physical and chemical characteristics of the environment. Early diagenesis, with major consequences on pore systems, is closely linked to the sequence stratigraphic context.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005