Advances in the origin of overpressures in sedimentary basins

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),velocitydensity 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 smectiteillite 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.

Prediction of abnormal pressure from AVO velocities over“Safety”field,onshore Niger delta,Nigeria

Abnormal pressure prediction was undertaken in“Safety”field,onshore Niger Delta,Nigeria using amplitude variation with offset(AVO)velocities information.Each of the methods used form an integral part of a process that produces AVO Analysis,AVO velocity inversion,extraction of seismic velocity from AVO velocities inversion results and pore pressure prediction.Pore pressure predicted from the seismic velocity has a better resolution than the pressure predicted from the interval transit time.The pore pressure within the field ranges from 14.7psi to 3916psi.Overpressured and underpressured zones were delineated on the field from the pressure predicted from the seismic velocities.Within the field,the overpressured zones were delineated at depth 6855 fte7802 ft.Over pressure top was delineated at a depth of 6855 ft with a pore pressure of 3053psi and a corresponding hydrostatic pressure of 2722psi.The under-pressured zones were also delineated at depth 7883 fte9288 ft.The under pressure top was delineated at a depth of 7883 ft with a pressure of 1093psi and a corresponding hydrostatic pressure of 3122psi.Porosity values within the over pressure zone ranges from 23%to 53%which could be considered as relatively high.This could be as a result of the fact that the pore fluid cannot be expelled rapidly thereby causing the pore fluid to increase rapidly since they are no longer compacted;thus leading to overpressure.As a result of overpressure top which is directly above the reservoir top within the shale zone,drilling this reservoir vertically could not be suggested so as to avoid possible blow out.It was also observed that the primary cause or mechanism of overpressure within this field could be disequilibrium compaction.