Hydrocarbon Enrichment Regularity of Nummulitic Limestone in Mediterranean Pelagian Basin

Pelagian Basin is located in Libya and Tunisia. Its onshore area accounts for 20% while the offshore area accounts for 80%. The basin is a meso-cenozoic superimposed basin. The cumulative oil/gas production reaches 528 MMBOE and the remaining recoverable reserves are 2441 MMBOE. The rate of proven resources is only 20%, so the exploration degree is relatively low. And the residual resources potential was great.

The Eocene series is the main petroleum-bearing combination in Pelagian basin. Its main reservoir is nummulitic limestone and the oil source rock is the Eocene mudstone. A large set of mudstone in upper Eocene, Oligocene, and Miocene is the main cap rock. Based on structural petroleum exploration as a guiding principle, the previous research found the structures of salt diapers and revealed the large-scale structural hydrocarbon reservoirs, such as Bouri oilfield, through the study on the reservoir growing area of the nummulitic limestone. But the drilling success ratio increasingly gets lower with the deepening exploration.

The authors propose that the Eocene reservoirs are controlled by the distribution of nummulitic limestone and the slope of the Jarrafa uplift is favorable to the lithologic hydrocarbon reservoirs of the nummulitic banks. The reservoirs are widely distributed and may form the large-scale reserves. Through the analysis on the key factors of reservoir formation, the distribution of nummulitic limestone reservoirs is chiefly controlled by paleodepth, paleotopography, and paleowind direction. The nummulitic reservoir develops well in the peripheral area of the paleoslope or the paleostructures. It easily forms lenticular reservoirs or updip pinchout lithologic hydrocarbon reservoirs. The water is shallow in the high position of the paleostructures while the water is deep in the lower position; these are unfavorable for nummulitic growth; then the reservoirs in them cannot develop. The success ratio aiming at such structures becomes increasingly low. The oil/gas enrichment conditions are met only in the structural traps in the dolomite developed area and structural traps caused by the late period salt diapir in nummulitic limestone developed area.

In conclusion, the Eocene nummulitic lithologic hydrocarbon reservoirs are the crucial exploration target.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California