Sarir Field - Structure

Structure

The Sirte Basin province ranks 13th among the world’s petroleum provinces, having known reserves of 43.1 billion barrels (6.85×109 m3) of oil equivalent (36.7 billion barrels (5.83×109 m3) of oil, 37.7 trillion cubic feet (1,070 km3) of gas, 100 million barrels (16,000,000 m3) of natural gas liquids). It includes an area about the size of the Williston Basin of the northern United States and southern Canada (≈490,000 km²). Late Mesozoic and Tertiary features developed on a Precambrian basement and eroded Paleozoic surface. The main northwest-to-southeast synclinal trough experienced repeated subsidence during fault adjustments. Several regional horst and graben trends originating in the Late Cretaceous remained active during the Tertiary as the Basin continued subsiding. The faults predominant trend is northwest-to-southeast, other northeast-to-southwest trends may form part of a conjugate pattern controlled by the gross texture basement.

In Pre-Cretaceous times, areas that became Sarir accumulations were occupied by topographic highs. It is probable that, even at this time, they were controlled by sets of conjugate faults trending northwest-to-southeast and northeast-to-southwest. The southern C structure may have been connected to a larger hinterland where Cretaceous sediments were derived and subsequently subsided.

Major fault movement occurred during Cretaceous sand deposition, most evident on north and west flanks of the C structure subject to uplift and erosion. This is expressed by a disconformity that removed successively deeper reservoir beds to the north and west. Major downwarping to the south probably began near the end of the Cretaceous period, isolating the C structure from its hinterland, forming it into a trap, and providing a deep shale trough that may have been a major hydrocarbon generating areas.

There was little fault movement during the Tertiary, but differential compaction created a simple anticline draped over the underlying Cretaceous structure. At basement level, Sarir C is less pronounced than structures to the south and northwest, which have poor sand development, in that Late Cretaceous shales rest on basement without the intervening sandstone reservoir of the Sarir field.

The fall from the Sarir C crest to the southern low is 1000 m, occurring over a 22 km distance equivalent to an overall dip of 2.5°. The steepest dip recorded is 4.5°. The triangular-shaped crest has an east-west base roughly 40 km long and a north-south perpendicular of 20 km. Vertical closure is 122 m. Sarir North and L-65 are on a northwest extension of the northeast side of the C structure. L-65, has a triangular shape with a southwest-trending flank. Structural evolution was marked by vertical tectonic movement with little evidence of horizontal stresses.