Advances in the origin of overpressures in sedimentary basins

Abstract

Much progress in the studies on overpressuring mechanisms has been made during the past one to two decades. (1) The causes of overpressure are divided into five categories, namely, disequilibrium compaction, fluid expansion, diagenesis, tectonic compression and pressure transfer. The fluid expansion involves hydrocarbon generation, oil cracking to gas and hydrothermal expansion. The diagenesis includes smectite-to-illite transformation. (2) Six methods for identifying overpressure origin are proposed, including log curves combination analysis, Bowers method (loading-unloading diagram), velocity-density crossplotting, correlation of porosities, pressure calculation and correlation, and comprehensive analyses. (3) With more and more application of empirical methods in the study of overpressure formation, almost all of the overpressure cases that are traditionally thought to be caused by disequilibrium compaction are denied totally or partly. Instead, the hydrocarbon generation is demonstrated to be the most significant mechanism for overpressure formation; the clay diagenesis (especially the smectite-illite transformation) as well as tectonic compression and pressure transfer are also important for overpressure formation. In addition, the overpressure formation in many basins is thought to be influenced by the combination of two or more overpressuring mechanisms. (4) Causes of overpressuring differ in lithology; for mudstones, the overpressure formation in source rocks is usually different from that of non-source rocks, the former of which is frequently related to hydrocarbon generation and sometimes also affected by diagenesis, while the later of which is commonly related to disequilibrium compaction, diagenesis and pressure transfer; for permeable rocks such as sandstones, overpressure is mainly caused by pressure transfer. (5) Because organic matter has an obvious influence on logging parameters such as density and acoustic velocity, an appropriate correction on the content of organic matter is needed when these logging data are used to analyze overpressure formation in organic-rich mudstones. It has been revealed that the cause of overpressuring based on the corrected log data can be quite different from that without correction.