Summary

Two different PETROLEUM SYSTEMS of UPPER JURASSIC to EARLY CRETACEOUS and LOWER-MIDDLE JURASSIC age, have been identified in the structurally controlled oil and gas Entre Lomas field in Neuquen Basin.

Entre Lomas Field is located in Neuquen Province, approximately 100 km north of the Neuquen City. It is part of a northwest-southeast regional structural trend, generated during the Late Triassic-Early Jurassic initial extensional strength on the eastern flank of Neuquen Basin.

The Entre Lomas structures are closely related with the movements of the master faults and were classified as a combination of fault forced folds on the footwall side and rollover folds on the hangingwall. Those faults are arranged in a transfer step pattern with antithetic and synthetic minor faults associated and well developed relay ramps in its extremes.

This important extensional fault complex was reactivated at different times keeping the same pattern which has conditioned the location of two different hydrocarbon accumulations on both sides of the master fault.

The oil accumulation is located on the footwall of Entre Lomas fault and is related with the UPPER JURASSIC to EARLY CRETACEOUS PETROLEUM SYSTEM. The source rock (Upper Jurassic black shales) has been tested by previous studies as oil prone. Despite it shows excellent generation conditions, its thermal maturity is moderate in the Entre Lomas field samples. Consequently the main reservoirs were probably filled through the migration from deeper and more mature source located right to the west.

The hangingwall gas accumulation is associated with a LOWER to MIDDLE JURASSIC PETROLEUM SYSTEM. There is not enough geochemical information of Lower Jurassic source rock in order to reach relevant conclusions. The samples obtained in Entre Lomas field and neighboring areas show gas prone conditions, but with variable maturity degree.

However, the presence of gas in clastic reservoirs in contact with the potential source allows the association of both in an independent system.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah