| Abstract | Wichian Buri oil field is the only oil field, which has produced petroleum from both clastic and fractured igneous reservoirs in Thailand. In this study, Rock Physics Templates (RPTs) were studied and constructed for characterization of these reservoir types. Two wells, WBEXT-14D and L44H-D2, were selected for the study. Three zones at depth 200-300 m, 400-500 m, and 1000-1100 m of well WBEXT-14D were selected to construct RPT for clastic reservoir, and one zone at depth 823-1036 of well L44H-D2 was used to construct RPT for fractured igneous reservoir. The results of well log interpretation of well WBEXT14D could not find out the HC zone as it is beyond the logging interval. The 200-300 m zone composes of Chaliang Lab formation and igneous unit rock A. The 400-500 m zone composes of Wichian Buri unit 1 and igneous rock unit B. The 1000-1150 zone composes of Wichian Buri unit 2 and igneous rock unit C. In this well, sandstone layers have GR ≈ 2540 API, RHOB ≈ 1.77-2.06 g/cc, Vsh ≈ 0.15-0.33, ϕn ≈ 0.56-0.78, and Sw ≈ 1, while the sandy shale layer has GR ≈ 45 API, RHOB ≈ 2.32 g/cc, Vsh ≈ 0.43, ϕn ≈ 0.47, and Sw ≈ 1. For well L44H-D2, fractured igneous rock unit C is the productive reservoir, the oil-bearing zone from 841 to 986 m has GR ≈ 47 API, RHOB ≈ 2.63 g/cc, Vsh ≈ 0.43, ϕn ≈ 0.17, Sw ≈ 0.4.
Regarding to RPT constructed in the first well, sandstones have VP/VS = 2.1-3.2 and PI = 0.0030.006 kg/m3*m/s*109. Sandy shales have VP/VS = 2.0-3.2 and PI = 0.004-0.007 kg/m3*m/s*109. Fractured igneous units A and B have VP/VS = 1.6-2.4, and PI = 0.007-0.012 kg/m3*m/s*109. RPTs for clastic rock zones were constructed using quartz with clay as main component for mineral moduli, while the rock physics models (RPMs) of HM-HS (Mavko et al., 1998) was used for calculation of dry moduli, and fluid substitution (Gassmann, 1951) was used for calculation of saturated moduli. For the second well of L44H-D2, the fractured igneous rock unit C has VP/VS = 1.5-2.2 and PI =0.012-0.016 kg/m3*m/s*109, while the intact igneous rock unit C has VP/VS = 1.6-2.1 and PI =0.015-0.018 kg/m3*m/s*109. The weathered igneous rock unit D and shale have VP/VS = 2.5-3.0 and PI =0.007-0.010 kg/m3*m/s*109, and VP/VS = 2.1-3.1 and PI =0.005-0.008 kg/m3*m/s*109, respectively. RPTs for igneous rock reservoir were constructed using the elastic bound model of Voigt–Reuss–Hill average (Hill, 1952) for mineral moduli calculation, the rock physics model of HM-HS for dry moduli calculation, and fluid substitution for saturated moduli calculation. The critical porosity varied, i.e., from 0.4, 0.2, 0.1, to 0.05, respectively. RPT of ϕc = 0.2 has the best match with WL data, and it can show the reservoir zone. These findings conformed that for construction of an RPT for a fractured igneous rock reservoir, the critical porosity has to be much smaller than ϕc = 0.4 as in the case of RPT constructed for the North Sea unconsolidated sediments as proposed by Ødegaard and Avseth (2013). |