Statistical analysis of subsurface temperatures in the Malay Basin has been carried out in order to (1) determine appropriate correction methods and correction factors for temperatures obtained from wireline logs and (2) investigate the confidence levels associated with those corrections. DST temperatures were taken as ‘ground truth’ for these corrections. For individual log-derived temperatures the correction factor f S , which is applied to the difference between the measured temperature and the surface temperature, is given by f s = ( − 0.1462 Ln ( TSC ) + 1.699 ) / ( 0 . 572 ⋅ Z 0.075 ) , where TSC is the time since end of mud circulation in hours and Z is the depth in metres. For temperatures that have already been corrected by extrapolation using Horner plots, f HP is given by f HP = − 0.1321 Ln ( TSC ) + 1.52 , where TSC is the maximum time since circulation stopped (hours) in the Horner plot set. Uncertainties in f S decrease markedly as TSC and depth increase. Uncertainties in f HP decrease as maximum TSC and the number of consistent temperature measurements at a given depth increase. Although these correction factors were developed using data from a single basin, our experience suggests that they can be used with reasonable confidence in many or most other geological provinces. Additional local calibrations would help test and refine this hypothesis.
Vitrinite-reflectance profiles for wells in the Malay Basin are generally consistent, and appear at first glance to accurately represent present-day thermal maturities. However, these measured Ro values are much lower than one would expect for wells with such high present-day geothermal gradients. Consequently, calculated Ro values can only be fitted to the measured Ro data by proposing a strong and recent heat pulse. In this scenario, the paleoheat flow was much lower than the present heat flow, and rose to the present levels within the last few million years or less. A plausible tectonic history for the Malay Basin can be constructed that justifies this scenario, because Quaternary volcanics and hot springs are known, and because the last 10 million years has seen renewed subsidence after a period of uplift during the Middle Miocene. However, FAMM (Fluorescence Alteration of Multiple Macerals) data obtained from seven wells indicate that the measured Ro values are much too low in most of the Malay Basin. Ro values have been suppressed by the presence of abundant liptinite and perhydrous vitrinite, probably as a result of marine influence, except along the western margin of the basin and in the far northwestern end. Calibration of the paleoheat flow with F AMM data permits use of a much more constant thermal history at each location. In this model, the main heat flow increased during Oligocene rifting in proportion to the amount of crustal extension, and then has subsequently decayed exponentially to modern levels. Using this paleoheat flow model, hydrocarbons are generated much earlier and maturities in the basin are much higher than if the paleoheat flow model is. calibrated using the measured Ro data. These conclusions in turn indicate that the recent tectonic history of the Malay Basin has probably been rather gentle, in keeping with evidence from sedimentation rates. Although we are not yet certain how common vitrinite suppression is globally or in the Malay Basin, these results indicate that (1) all data sets should be routinely checked for vitrinite suppression, especially in areas where the phenomenon has been recognized; (2) any thermal model requiring a significant recent heat pulse to match measured and calculated Ro values should be viewed with suspicion until validated independently; and (3) errors in reconstruction of thermal and tectonic history can often lead to significant errors in exploration decisions.
Data from subsurface temperature measurements provide widely used and vital input data for maturity modeling_ Because maturity calculations are very sensitive to thermal history, and because reconstruction of the thermal history begins with the modern temperature profile, accurate knowledge of true formation temperatures is vital.Temperature data used in maturity modeling come from a variety of sources, including BHTs derived from single logging runs, BHTs obtained from multiple logging runs at the same depth and corrected using the Horner plot method, RFTs, DSTs, and production tests (PTs)_ However, temperatures obtained using most of these techniques require some correction before they represent true formation temperatures. Unfortunately, the need for these corrections is not generally recognised, leading most modelers to consistently underestimate modern subsurface temperatures_ Such errors can lead to major errors in subsequent calculations of hydrocarbon generation and cracking, and can thus have profound effects on exploration decisions_In an effort to evaluate the accuracy of data from single logging runs, Horner plots, RFTs, and DSTs or PTs, an extensive temperature data base was developed for the Malay Basin. Basal heat flows calculated for many wells using each type of temperature data were compared. It was found that all other temperature data considerably underestimated subsurface temperatures compared to DSTIPT data, which were assumed to represent true formation temperatures. Results were analyzed using two different statistical approaches, which gave quite consistent conclusions, and average correction factors for each type of temperature data were developed. To be equivalent to heat flows calculated from DSTI PT temperatures, heat flows calculated using single BHT data already subjected to a standard 10% correction had to be corrected upward by an additional 16%, those calculated from Horner plot extrapolations by an additional 14%, and those obtained from uncorrected RFT data by 9%.Measured subsurface temperatures were corrected using a more complex set of equations that take surface temperature (T.) into account. The corrected subsurface temperature To is given by one of the following formulas, where Tb is the uncorrected temperature from a single logging run, Th is the extrapolated temperature obtained from a Horner plot, and Tr is the uncorrected RFT temperature. To = (1.1 e T b -T.)e1.16 + T. To = (T h -T.)e1.14 + T. To = (T r -T.)e1.09 + T.Although these correction factors were developed for the Malay Basin, evidence presented by other workers suggests that corrections are needed in other basins as well, and that the magnitude of the corrections suggested here is reasonable for other areas. Future work should test these hypotheses and extend this calibration to ·other types of basins in other parts of the world.
Based on carbon-isotope ratios for CO 2 , methane, ethane, and propane, and on CO2 contents and the relative proportions of methane, ethane, and propane, we have identified three end-member gas types in the Malay Basin: biogenic gas, thermal gas, and basement gas. The thermal gas has been divided into two subgroups: "normal" thermal gas originating at relatively shallow depths, and "deep" thermal gas from more-mature source rocks. Most gas samples studied in the Malay Basin are composed of mixtures of two or all three of the e~d members. Gases with a significant biogenic component are limited to the northeast corner of the b~sm, and are not associated with large accumulations. The biogenic gas was probably generated locally smce the end of the Middle Miocene, and does not appear to offer an important exploration target in the Malay Basin. Gases dominated by CO 2 are predominantly sourced from the basement. They are found along a discontinuous trend from Dulang to mar, and along another from Bunga Raya to Bunga Pakma. Because these gases have migrated vertically from the basement, they dominate only where extensive fault systems extend all the way to the basement. Although some accumulations along this trend are very large, targets are at risk of being dominated by CO 2 , with risk increasing with increasing proximity to basement. The area in the north central part of the basin contains gas that appears to be mainly of "normal" thermal origin. Accumulations are of moderate size. Lack of contamination by basement gas and "deep" thermal gas in this area suggests a lack of deep faults. Lack offault-related vertical migration pathways will limit the volume of hydrocarbon gas in this area, and thus downgrades its exploration potential, except where there is local evidence for deep vertical faults. Much ofthe basin contains gas that is a mixture of "normal" thermal hydrocarbons, "deep" thermal hydrocarbons, and CO 2 from the basement in varying proportions. "Deep" thermal gas seems to dominate over "normal" thermal gas in the large accumulations, suggesting that the key to exploring for large gas reserves is to find areas where vertical faults are adequate to drain the deep strata responsible for generating the large volumes of "deep" thermal gas, but where there is also evidence that these faults do not extend all the way to the basement. The region between Damar and Tujoh, where large reserves are present with only moderate amounts of CO 2 , may serve as a model for this type of migration. Integration of these data with analysis of structural styles should provide important guidelines for future gas exploration in the Malay Basin.
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