Abnormal pressure strata are believed to exist throughout the Malay Basin with the exception of the southwest and northeast margins. The onset of abnormal pressure is abrupt in the northeastern portion of the basin but is generally more gradual in the southeastern area.
The abnormal pressure is a complex origin and is attributed to a combination of rapid burial of certain stratigraphic units, uplift of initially normally pressure strata, faulting and hydrocarbon column effects.
Predrill abnormal pressure predictive techniques used by EPMI in the Malay Predrill abnormal pressure predictive techniques used by EPMI in the Malay Basin are interpretation of seismic interval velocities and electric log correlation. With accurate seismic velocity information, an interval transit time profile, called a Pennebaker plot has aided in predicting top of abnormal pressure 80% of the wells drilled to date. Log correlations is most applicable within the confines of a particular structure.
Detection of abnormal pressure in the Malay Basin while drilling is difficult at standard pressure indicators while drilling and use of industry generated overlays to derive quantitative estimations of pore pressure from drilling exponent plots do not provide satisfactory results. pressure from drilling exponent plots do not provide satisfactory results. As a result, EPMI developed a "Malay Basin" overlay which provides more accurate quantitative estimation of pore pressure from drilling exponents. In addition, EPMI employs a technique for monitoring gas-feed in, called an extended connection, or 10-10-10 to detect geopressure while drilled.
Abnormal pressure is a condition of subsurface formations whereby the pressure exerted by the fluids within a rock unit at a particular depth exceeds the hydrostatic pressure exerted by a particular depth exceeds the hydrostatic pressure exerted by a column of water equivalent to that depth. The accurate prediction and detection of abnormal pressure are prerequisites for wells to be drilled safely such that the pressure exerted by the formation fluids.
In Tertiary sediments of the Malay Basin, a major young sedimentary basin which lies offshore the east coast of Peninsular Malaysia (Figure 1) abnormal pressure has been encountered in two third of the structures drilled by Esso Production Malaysia Inc. Well control problems have occurred during Esso's twelve-year exploration drilling history but fortunately no personnel injuries nor damage to either drilling equipment or the environment has occurred. There has, however, been loss of technical data as portions of several wells were not completely evaluated prior to portions of several wells were not completely evaluated prior to abandonment because abnormal pressure and well control problems were encountered. problems were encountered. The following discussion summarizes Esso's current concepts of the occurrence and causes, and the methods of predicting and detecting abnormal pressure in the Malay Basin. The data base from which conclusions on its occurrence and causes are drawn is presently limited to approximately 150 exploration wells and 130 development wells. Prediction and detection methods, which include both standard industry procedures plus techniques developed locally by Esso, are sophisticated, but due to a variety of factors discussed in this paper, are not as successfully applicable in the Malay Basin as they are elsewhere.
Based upon results of drilling to date abnormally pressured strata are believed to exist throughout the entire Malay Basin with the exception of the southwest and northeast where the stratigraphy is dominated by massic Miocene sands (Figure 2). The onset of abnormal pressure within the sedimentary section normally occurs in progressively older units both from northwest to southeast along the axis of the basin and from the center to the flanks of the basin. The stratigraphically youngest top of abnormal pressure is in D Group siliciclastics of late Miocene age on isolated pressure is in D Group siliciclastics of late Miocene age on isolated structural highs in the center of the basin while the stratigraphically oldest fines occurrence is in the M Group units of early Oligocene age in the basin's southeastern regions. Abnormal pressure in Groups D to I (Late to early Miocene) is typically encountered at subsea structural depths ranging from 4000 to 7000 feet, and in Groups J, K, L, and M (Oligocene to lowe Miocene) from 4500 to 8000 feet.
The onset of abnormal pressure is abrupt in the northwestern portion of the basin with mud weight equivalent pressures typically portion of the basin with mud weight equivalent pressures typically increasing from 8.5 pounds per gallon (ppg) to 13 ppg in 500 to 700 hundred feet. Pressure increases are generally more gradual in the southeast area of the basin where the equivalent 8.5 ppg to 13 ppg mud weight equivalent increase characteristically occurs in about 900 hundred feet. Formation pressures up to approximately 16.5 ppg (+0.85 psi/ft) have been penetrated and pressures of 14 ppg ppg (+0.85 psi/ft) have been penetrated and pressures of 14 ppg (0.7 psi/ft) are routinely encountered.
Abnormal pressure in the Malay Basin is apparently of complex origin and is attributed to various combinations of rapid burial of certain stratigraphic units, uplift of initially normally pressured strata, faulting and hydrocarbon column effects. While the relative importance of these factors materially varies both geographically and stratigraphically, the specific ways in which they interplay on discrete structures to produce the observed pressure regimes is complex and not well understood. The effect of recharge is a factor of uncertain but potential importance.
Rapid burial of the mid to late Miocene section appears to be the dominant regional cause of abnormal pressure in the northeastern and central portions of the Malay Basin. Siliciclastics of Groups D to H were deposited there in nearshore to non-marine environments. Silty shales and sandstones and coals comprising the remainder of the section. These sediments were initially buried very quickly, thereby restricting fluid expulsion. Continued burial partially compacted the section, which coupled with the presence of partially compacted the section, which coupled with the presence of laterally discontinuous permeable sandstones incapable of dissipating the formation waters, resulted in the development of abnormal pressure.