Development of Abnormally High Pore Pressures in a Geologically Young, Basin-Centered Oil and Gas Accumulation, Mako Trough, Hungary
The processes involved in the development of abnormally high pressured basin-centered gas and oil accumulations (BCGA’s/BCOA’s) have been formulated on observations of Paleocene and older accumulations from North America that have experienced temperatures and depths of burial greater than presently observed. As a consequence, the development of abnormal pore pressures has been the subject of controversy. One model maintains that overpressures are a consequence of hydrocarbon generation during maximum burial and temperature. Alternatively, another model proposes the development of overpressuring as a consequence of structural uplift. The opportunity to test these hypotheses is provided by an Upper Miocene and Pliocene BCGA/BCOA in the Mako Trough, southeastern Hungary.
Structurally, the Mako Trough is a sub-basin within the larger Pannonian Basin of central Europe that has not experienced significant vertical uplift. The thickness of the BCGA/BCOA is approximately 2,500 m. Fluvial, deltaic, and lacustrine environments of deposition are represented. Source rocks occur primarily within Pannonian and younger Miocene rocks and the organic matter ranges from types I to III kerogen, thus, both oil and gas have been generated. Reservoirs typically are sandstone with low-permeability (<0.1md) and porosity (< 14%). Heat flow values are as high as 100 mW/m2. As a consequence of the high heat flow, temperature at depths greater than 5,500 m (18,040 ft) are high (460°F) and levels of thermal maturity are also high, attaining vitrinite reflectance values of 2.0%. Pressure gradients at these depths are as high as 1.0 psi/ft. As a consequence of the high temperatures in the Mako Trough the petroleum system is dynamic and source rocks are actively generating and charging reservoirs, unlike current conditions in most North American BCGA’s/BCOA’s.
The dynamic nature of the hydrocarbon system in the Mako Trough has resulted in the development of pore pressures in excess of hydrostatic pressure. The original mechanism of overpressure may have been compaction disequilibrium that was subsequently replaced by a hydrocarbon generation mechanism during maximum burial; structural uplift is not required for the development of overpressure. The development of overpressures through a hydrocarbon generation mechanism in this geologically young sequence provides a good analog for the incipient development of abnormally high pressures in BCGA’s/BCOA’s.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009