Bill Ambrose, Research Associate
Bureau of Economic Geology, The University of Texas at Austin

ABSTRACT
The Macuspana Basin contains a large undiscovered gas resource in 30 siliciclastic Upper Miocene and Pliocene gas plays. This amount exceeds the produced gas (5.3 Trillion cubic ft [Tcf]) and recently discovered gas in the basin. Four plays contain half of these undiscovered gas resources. Many other plays are tested in limited areas and have great potential for additional exploration.

This play analysis of the Upper Miocene and Pliocene provides a detailed framework for future exploration by integrating geological, engineering, petrophysical, and production data to establish and rank proven and hypothetical plays. This approach uses reservoir recovery efficiency data from existing fields and risk factors such as reservoir presence and quality, trap, seal, source, and migration, as well as average-prospect chance.

This study includes a structural and stratigraphic analysis of the Upper Miocene and Pliocene section to map regional controls on play distribution. Major structural controls are deep-seated faults that tap Mesozoic thermogenic gas sources, areas of intense shale diapirism and folding, and areas prone to structural inversion that enhance trapping and reservoir productivity. The primary stratigraphic controls are regional facies distribution of the major productive stratigraphic intervals.

Exploration strategies for each play should consider the range in risk elements specific to each play. Reservoir presence and quality are risk elements for undeveloped Upper Miocene deep-water plays, where reservoir lithology is unknown. Knowledge of the deep-seated fault systems will enable targeting areas with Mesozoic sources. Future exploration in mature onshore plays will benefit from aggressive acquisition of 3-D seismic data to improve imaging and evaluation of untested structural closures.