COMPTON, PAUL, ALI HIMMALI, and RICHARD WALKER, LASMO Grand Maghreb Ltd

Abstract: The Geology of the Elephant Field, Murzuk Basin, Libya

The Elephant oil field was discovered in October 1997 in the Murzuk Basin of SW Libya by a consortium of LASMO, KNOC and AGIP. The discovery well, F1-NC174, encountered a gross oil column of 113m, and was tested at a cumulative rate of 7500 bopd of 38 degrees API crude. After extensive seismic and appraisal well programmes, reserves are estimated to be in excess of 500 MMstbo, which means that Elephant is the largest discovery to have been made in Libya for 13 years.

The reservoir comprises high net/gross ratio quartz and subarkosic arenites, of Late Ordovician age belonging to the Memouniat Formation.These sandstones are interpreted as having been deposited mainly as turbidites around the margins of an extensive ice sheet. During subsequent diagenesis, the primary porosity has been partially occluded by extensive quartz overgrowths, but enhanced by partial dissolution of potash feldspar.This dissolution has particularly enhanced the permeability of many layers, with the result that the reservoir properties over most of the field are excellent.

The source for the oil is believed to be the Lower Silurian Tanezzuft Formation, a thick shale unit, which is also the top seal for the accumulation.The most organically rich part of the formation is a radioactive shale at the base, which is present over the northern part of the field. Regionally,TOC's up to 17% are recorded for this hot shale.

The oil is trapped in a reverse faulted anticline, formed over a basement uplift of compressional origin, with dip closure to the north, east and south.The structure resulted from several pulses of uplift.An extensive 3D survey has been acquired to define more closely the field structure, investigate seismic stratigraphy within the reservoir and provide further velocity control for depth conversion.

This paper describes the field, placing it in its regional context, and detailing some of the studies employed in maximising the efficient and rapid development of the reserves.

AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England