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AAPG Bulletin, V.
Evidence for overpressure generation by kerogen-to-gas maturation in the northern Malay Basin
Mark R. P. Tingay,1 Chris K. Morley,2 Andrew Laird,3 Orapan Limpornpipat,4 Kanjana Krisadasima,5 Suwit Pabchanda,6 Hamish R. Macintyre7
1Tectonics, Resources and Exploration, Australian School of Petroleum, University of Adelaide, Adelaide, South Australia 5005 Australia; [email protected]
2PTT Exploration and Production Public Company Limited, 555 Vibhavadi-Rangsit Road, Bangkok 10900 Thailand; [email protected]
3CEC International Ltd (Thailand Branch), Units 3901–3904, 39th Floor, Exchange Tower, 388 Sukhumvit Road, Klongtoey, Bangkok 10110 Thailand; [email protected]
4PTT Exploration and Production Public Company Limited, 555 Vibhavadi-Rangsit Road, Bangkok 10900 Thailand; [email protected]
5PTT Exploration and Production Public Company Limited, 555 Vibhavadi-Rangsit Road, Bangkok 10900 Thailand; [email protected]
6PTT Exploration and Production Oman Company Limited, P.O. Box 1067, P.C. 133, Al Khuwair, Muscat, Oman; [email protected]
7BG-Group Global Technology Centre BG Brazil, Av. Republica do Chile, 330-25th floor Torre Oeste Centro, 20031-170, Rio de Janeiro, Brazil; present address: BG-Group Global Technology Centre BG Brasil, Av. Republica do Chile, 330-25th Floor Torre Desk Centro, 2003 1-170, Rio de Janerio, Brazil; [email protected]
Gas generation is a commonly hypothesized mechanism for the development of high-magnitude overpressure. However, overpressures developed by gas generation have been rarely measured in situ, with the main evidence for such overpressures coming from source rock microfractures, the physical necessity of overpressures for primary migration, laboratory experiments, and numerical modeling. Indeed, previous in-situ observations suggest that gas generation only creates highly localized overpressures within rich source rocks. Pore-fluid pressure data and sonic velocity–vertical effective stress plots from 30 wells reveal that overpressures in the northern Malay Basin are primarily generated by fluid expansion and are located basinwide within the Miocene 2A, 2B, and 2C source rock formations. The overpressures are predominantly associated with gas sampled in more than 83% of overpressure measurements and have a sonic-density response consistent with gas generation. The association of fluid expansion overpressures with gas, combined with the sonic-density response to overpressure and a regional geology that precludes other overpressuring mechanisms, provides convincing in-situ evidence for basinwide gas generation overpressuring. Overpressure magnitude analysis suggests that gas generation accounts for approximately one-half to two-thirds of the measured excess pore pressure in the region, with the remainder being generated by coincident disequilibrium compaction. Thus, the data herein suggest that gas generation, if acting in isolation, is producing a maximum pressure gradient of 15.3 MPa/km (0.676 psi/ft) and not lithostatic magnitudes as commonly hypothesized. The gas generation overpressures in this article are not associated with a significant porosity anomaly and represent a major drilling hazard, with traditional pore-pressure prediction techniques underestimating pressure gradients by 2.3 1.5 MPa/km (0.1 0.07 psi/ft).
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