Changes

==Phase I==

==Phase I==

===Direct and indirect systems===

===Direct and indirect systems===

During the early burial and thermal histories of direct and indirect systems, the reservoirs are, for the most part, normally pressured, and the fluid phase in the pore system is 100% water saturated ([[:file:BasinCenteredGasFig1.jpg|Figure 1]]). Compaction of framework grains during this phase is an important process. The defining processes for each system, however, are different. For direct systems, phase I terminates with the initiation of thermal gas generation, whereas the termination of phase I in indirect systems occurs with the initiation of thermal cracking of oil to gas. Reservoir quality in indirect systems during phase I is assumed to be relatively better than reservoir quality in direct systems because buoyant accumulations of oil require better porosity and permeability.

During the early burial and thermal histories of direct and indirect systems, the reservoirs are, for the most part, normally pressured, and the fluid phase in the pore system is 100% water saturated ([[:file:BasinCenteredGasFig1.jpg|Figure 1]]). Compaction of framework grains during this phase is an important process. The defining processes for each system, however, are different. For direct systems, phase I terminates with the initiation of thermal gas generation, whereas the termination of phase I in indirect systems occurs with the initiation of thermal [[cracking]] of oil to gas. Reservoir quality in indirect systems during phase I is assumed to be relatively better than reservoir quality in direct systems because buoyant accumulations of oil require better porosity and permeability.

During phase I there may be some cases in which reservoir pressures are overpressured. Law and Spencer<ref name=Lawandspencer_1998>Law, B. E., and C. W. Spencer, 1998, [http://archives.datapages.com/data/specpubs/memoir70/m70ch01/m70ch01.htm Abnormal pressure in hydrocarbon environments], ''in'' B. E. Law, G. F. Ulmishek, and V. I. Slavin, eds., Abnormal pressures in hydrocarbon environments: [http://store.aapg.org/detail.aspx?id=751 AAPG Memoir 70], p. 1-11.</ref> suggested that in the early burial stages of a basin-centered gas accumulation (BCGA) sequence, prior to the development of a recognizable BCGA, and in some depositional settings of rapid sedimentation, compaction disequilibrium may have been the initial overpressuring mechanism. In this scenario, the pressuring fluid phase is water. However, as the sequence experiences further burial and hotter temperatures, the compaction disequilibrium pressure mechanism may be replaced by hydrocarbon generation and the development of abnormally high pressures characterized by pore fluids composed of gas and little or no water. A possible example of the transition of pressure mechanisms from compaction disequilibrium to hydrocarbon generation may be present in [[Miocene]] and [[Pliocene]] rocks in the Bekes basin<ref name=Spenceretal_1994>Spencer, C. W., A. Szalay, and E. Tatar, 1994, Abnormal pressure and hydrocarbon migration in the Bekes basin, ''in'' P. G. Teleki, R. E. Mattick, and J. Kokai, eds., Basin analysis in petroleum exploration: Dordrecht Netherlands, Kluwer Academic Publishers, p. 201-219.</ref> and the Mako trench (B. E. Law, 2000, unpublished data) of Hungary. In these areas, Miocene and Pliocene rocks are overpressured and possess many of the distinguishing characteristics of a BCGA. The overpressures in Miocene rocks appear to be caused by hydrocarbon generation, whereas overlying, overpressured Pliocene rocks appear to be in a transitional pressure phase between compaction disequilibrium and hydrocarbon generation. In this case, a knowledge of pore fluid composition (mainly gas or mainly water) in the Pliocene sequence would offer considerable insight in resolving the problem.

During phase I there may be some cases in which reservoir pressures are overpressured. Law and Spencer<ref name=Lawandspencer_1998>Law, B. E., and C. W. Spencer, 1998, [http://archives.datapages.com/data/specpubs/memoir70/m70ch01/m70ch01.htm Abnormal pressure in hydrocarbon environments], ''in'' B. E. Law, G. F. Ulmishek, and V. I. Slavin, eds., Abnormal pressures in hydrocarbon environments: [http://store.aapg.org/detail.aspx?id=751 AAPG Memoir 70], p. 1-11.</ref> suggested that in the early burial stages of a basin-centered gas accumulation (BCGA) sequence, prior to the development of a recognizable BCGA, and in some depositional settings of rapid sedimentation, compaction disequilibrium may have been the initial overpressuring mechanism. In this scenario, the pressuring fluid phase is water. However, as the sequence experiences further burial and hotter temperatures, the compaction disequilibrium pressure mechanism may be replaced by hydrocarbon generation and the development of abnormally high pressures characterized by pore fluids composed of gas and little or no water. A possible example of the transition of pressure mechanisms from compaction disequilibrium to hydrocarbon generation may be present in [[Miocene]] and [[Pliocene]] rocks in the Bekes basin<ref name=Spenceretal_1994>Spencer, C. W., A. Szalay, and E. Tatar, 1994, Abnormal pressure and hydrocarbon migration in the Bekes basin, ''in'' P. G. Teleki, R. E. Mattick, and J. Kokai, eds., Basin analysis in petroleum exploration: Dordrecht Netherlands, Kluwer Academic Publishers, p. 201-219.</ref> and the Mako trench (B. E. Law, 2000, unpublished data) of Hungary. In these areas, Miocene and Pliocene rocks are overpressured and possess many of the distinguishing characteristics of a BCGA. The overpressures in Miocene rocks appear to be caused by hydrocarbon generation, whereas overlying, overpressured Pliocene rocks appear to be in a transitional pressure phase between compaction disequilibrium and hydrocarbon generation. In this case, a knowledge of pore fluid composition (mainly gas or mainly water) in the Pliocene sequence would offer considerable insight in resolving the problem.