MODELS OF POROSITY FORMATION AND THEIR IMPACT ON RESERVOIR DESCRIPTION, LISBURNE FIELD, PRUDHOE BAY, ALASKA

The Lisburne field at Prudhoe Bag: Alaska, produces from shelfal carbo nates in the Pennsylvanian Wahoo formation. Four major factors control reservoir behavior: (1) depositional stratification, (2) a fractured, permeable subunconformity alteration zone (SAZ), (3) multiple episode s of porosity formation, and (4) faulting. This paper is the first wri tten description of facies, cyclicity, and diagenetic processes as the y apply to porosity formation and reservoir modeling in the Lisburne f ield. Successive depositional cycles in the Wahoo formation pass from ooid/skeletal grainstones deposited in shoal complexes to oncolitic pa ckstones and skeletal/peloidal wackestones formed in restricted lagoon al environments. Geochemical data and crosscutting relationships betwe en porosity and unconformities, pressure solution features, fractures, and faults provide evidence of three distinct episodes of porosity fo rmation. Earliest porosity is probably due to periodic, localized expo sure during the Pennsylvanian. A second stage of porosity is associate d with shallow-burial dolomitization that probably began during Permia n-Triassic subaerial exposure. Reservoir quality in dolomites varies w ith the degree of neomorphic recrystallization. The third stage began in the Cretaceous and lasted into the Tertiary, and is associated with final burial and hydrocarbon maturation. This burial dissolution even t also opened existing fault systems, creating a complex reservoir. Mo st Lisburne field effective porosity is of late-burial origin due to e ither dolomitization or dissolution. Faults and the SAZ act as giant c ollectors of oil from low-permeability matrix, and they have reduced t he number of wells needed for field development. Faults also complicat e waterflood implementation and maintenance of uniform reservoir press ure. Field studies such as these demonstrate the importance understand ing the diagenetic history of a reservoir can have for field managemen t and development planning.