Facie-Guided 3-D Seismic Modeling: Part 1, Model Construction

A realistic, reservoir-scale, 3-dimensional earth model of the lower Miocene (Fleming) sandstone reservoir sequence at Powderhorn field, Calhoun County, Texas has been constructed. We first mapped the detailed lithology of 250 m of stratigraphic section over an area of 16 by 11 km. Wireline logs from more than 110 wells provided the data for a detailed facies analysis of 14 sandy depositional units encased in 15 shale units. The average thickness of mapped sand units is 3-30 m. In six wells, porosity and shaliness were evaluated from density, neutron, and spontaneous potential (SP) logs. These parameters correlate well to acoustic and density logs values. Based on relationships from these wells, key rock properties were estimated from SP and resistivity logs in older wells with incomplete log suites. These properties were then interpolated between wells within a 13 by 9 km model area. Interpolation incorporated the facies analysis through a 2-dimensiona1, directional computer mapping algorithm. 3-dimensional porosity, shaliness, P-wave velocity, density, and impedance models were constructed. The P-wave velocity and impedance models were combined to produce primary reflection coefficient sequences. Convolution with a zero-phase wavelet has generated perfectly migrated, wavelet-processed, noise-free, 3-dimensional synthetic seismic data. These models are geologically realistic and have appropriate stratal complexity at reservoir scale. The models are used to test the capability and limitations of 3-dimensional seismic data interpretation in an accompanying paper. sandstones. Facies analysis of each of the 14 sandy depositional units incorporated SP log-deflection patterns, inferred sand body geometry, and a limited amount of petrological information. Three major depositional systems have been recognized in the study area: barrier bar-lagoon, coastal streamplain and bayhead delta, and wave-dominated delta. Barrier bar-lagoon. This system is characterized by thick, strike-elongate, barrier-core sandstones that are transected by sandy, dip-oriented, inlet-fill deposits. Floodtidal-delta sandstones abut the landward side of the barrier core, and pinch out landward into lagoonal shale (Figure 2a). Coastal streamplain and bayhead delta. These sandy units were deposited in the coastal streamplain that existed landward of the barrier bar-lagoon system (Figure 2b). The small, dip-elongate fluvial channels filled with thin, composite sand bodies that amalgamated into broader, sometimes thicker sand belts. The channel belts were flanked by levee deposits of sand and silt, and thickened crevasse splays. Seaward, these fluvial channels frequently terminated as bayhead deltas that prograded into the shallow water lagoon. Wave-dominated delta. Thick, strongly strikeoriented sand bodies transected by dip-elongate channel belts were deposited in small wave-dominated deltas (Figure 2c). The dip-oriented channels are distributaries that are flanked by areally-restricted sand-prone levees and crevasse splays, and by sand-poor lake deposits in the delta plain. Basinward, beach-ridge sandstones were reworked by waves from mouth bar deposits.