Gulf of Mexico overpressure and clay diagenesis without unloading: An anomaly?

Abstract

ABSTRACT Compaction disequilibrium is a widely accepted cause of overpressure, especially in clay-rich, rapidly deposited sediments. Clay diagenesis has been associated with the occurrence of overpressure greater than the compaction disequilibrium overpressure. These observations have led to the expectation that overpressure will be greater than the compaction disequilibrium contribution when clay diagenesis occurs within an overpressured mudstone. Clay diagenesis have been reported in a Pliocene section of a well from the Gulf of Mexico, offshore Louisiana. Pressure and log data from that well indicate that despite clay diagenesis, the overpressure can be attributed solely to compaction disequilibrium. This paper examines the whole mudstone and clay mineralogy composition and petrophysical characteristics of the offshore Louisiana well with clay diagenesis, but without a diagenesis contribution to overpressure and contrasts that data with results from other clay diagenesis and petrophysical studies. The comparison suggests that the offshore Louisiana well was relatively smectite poor compared with wells from regions associated with a clay diagenesis contribution to overpressure. The lower smectite content resulted in a lower percentage of reacted volume that was insufficient to allow the load transfer often associated with clay diagenesis. Petrophysical features of the offshore Louisiana well and nearby wells differ from the features associated with clay diagenesis in other Gulf of Mexico wells and a limited number of international wells. Comparison of location, age, depositional package, clay mineralogy, and petrophysical features suggests that provenance may control the occurrence of Gulf of Mexico mudstones that do not experience increased overpressure as a result of clay diagenesis.