U.S.A., Miocene carbonate reservoirs may contain appreciable amount of microporosity in grain, cement or matrix. Depending on the crystallometry and morphometry of micrite particles, microporosity can vary both in amount and in nature. Different kinds of micrite microtextures may produce different types of microporosity. This study proposes a classification of micrite microtextures and microporosity. Four different types of micrite microtextures are classified in this study; these are rounded micrites, subrounded micrites, microrhombic and polyhedral micrites, and compact anhedral micrites. Four different types of micropores are classified based on their pore sizes. These are very fine, fine, medium, and coarse micropores. The classification of microporosity allows its occurrence in carbonate reservoirs to be explained depending on micrite microtextures. The classification of micropores in terms of their micrite microtextural occurrence lead us to define the micropores that are contributing to the permeability of the reservoirs and allow fluid flow in carbonates. The classes of micropores together with the corresponding micrite microtextures of their occurrences help to define the potential reservoir rocks in terms of micrite microtextures.
Summary
This study demonstrates the use of wireline logs for the overpressure-mechanisms analysis in a field in the southwestern Malay basin. The development of overpressure means that the fluid movement in the pores is retarded, both vertically and laterally. In many Tertiary basins, overpressure is mainly generated by compaction disequilibrium caused by a high deposition rate and low permeability in shales. In the Malay basin, temperature and high-heat flow also play an important role in generating overpressure at a shallow depth, because the geothermal gradient is very high (40–60°C/km). Pore-pressure profiles and crossplots of sonic velocity/vertical effective stress and of velocity/density are used to derive the overpressure-generating mechanisms. The results obtained from the crossplots of 10 wells reveal that in the study area, overpressure is generated by both primary (compaction-disequilibrium) and secondary (fluid-expansion) mechanisms. The overpressure-magnitude analysis suggests that the overpressure generated by the secondary mechanism is very high compared with the primary mechanism. In all the wells, the Eaton (1972) method with an exponent of 3 gives good prediction when overpressure is the result of the compaction-disequilibrium mechanism, but it underpredicted the high pore pressure where the fluid-expansion mechanism is also present. However, by use of a higher Eaton exponent of 5 for the fluid-expansion mechanism, the overpressures are predicted quite well. The Bowers (1995) method, by use of the unloading parameter (U) of 6, is also used for pressure prediction and it gives a reasonably good prediction in the high-overpressure zone of all the wells.
Microporosity can occur in grain, cement or matrix and it is related to the depositional and diagenetic history of the carbonate reservoir. Depending on the crystallometry and morphometry of micrite particles, microporosity can vary both in amount and in nature. Different kinds of micrite microtextures may produce different kinds of microporosity. This study proposes a classification of micropores and micrite microtextures. We propose a classification that includes four micrite microtextures in Miocene Carbonate reservoirs, offshore Sarawak, Malaysia. Rounded, subrounded, microrhombic and polyhedral, and compact anhedral micrites have their distinct crystal morphology and crystal contacts. This microtexture classification of micrite particles can explain the occurrence, distribution and abundance of microporosity in Miocene carbonates. Based on the crystallometry, morphometry and textural arrangements of micrites a classification scheme is proposed that includes four different classes of micropores such as very fine, fine, medium and coarse micropores with a maximum pore diameter of 0.1 to 2 micrometer, 2 to 4 micrometer, 4 to 6 micrometer and 6 to 10 micrometer respectively. Microporosity affects the empirical porosity-permeability relationship in carbonate reservoirs and influences the assessment of ultimate recovery of hydrocarbon. The application of microtextural classification of microporosity can explain the scattering of data points in porosity-permeability crossplot.
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