Abstract

GeoSpectrum was contracted by Burlington Resources to map in detail a major basinal turbidite sandstone reservoir. The reservoir interval for the gas-containing sandstones varies in thickness from 800 to 1100 feet. Based on normalized gamma ray logs for 349 wells, a sequence of sandstone packages was correlated across a leasehold of approximately thirty square miles north and west of the town of Sonora (Figure 1; after Hamlin et al., 1995).

Maps and cross-sections indicate that the four sandstone packages, called the C, D, E and F Units, have their own characteristics that change in time and space. The sandstone units are identified as a sequence of related packages in the sense of mappable informal formations rather than as individual beds. A combination of computer technology and the old-fashioned “slipping of the paper logs” was employed to correlate one well log to another. Standard colored plots of digital curves for each well were devised in geoPLUS’s Petra software at a scale of 1 inch equals 100 ft. The routine work of sand correlation was done with colored paper strip logs, in which each well could be compared to its neighbors. The correlation was then extended to reference dip and strike lines across the entire leasehold area. Once cross-correlated, the footage tops for each of the zones were penciled-in at the appropriate horizon on the well strips and entered back into the Petra software for mapping purposes.

During a major lowstand in earliest Permian time, these sandstone complexes were transported across the shelf and slope and were deposited by turbidity currents to a lowermost slope or basinal setting. The source areas of the sandstones transect the Eastern Shelf to the northeast of the study area. In the distal portion of the system, down dip to the southwest and into the Val Verde Basin, the sandstones grade into basinal dark mudstones. Variations in Bouma sequences in core descriptions (Figure 2; after Walker, 1979 and Figure 3; after Hamlin et al., 1995) reported in the literature indicate lithofacies associations consistent with deposition by turbidity currents forming a submarine fan. The depocenters, or the middle-fan position, of Units C, D, and E migrate progressively updip to the northeast whereas the more complex depocenters of Unit F recede downdip to the southwest. The sandstones are part of vertically-stacked, inner-fan channel deposits or coalescing middle-fan lobes, depending on location in time and space for each of the sandstone units.

The coalescing middle-fan deposits at the depocenters of C and D Units sands are clean sandstone-on-sandstone stacks and the composite might act locally as one thick reservoir unit. The channelized inner-fan deposits updip of the depocenters are more separated by shales and clayey sandstones, which should make for complex but rather restricted reservoir units.

As a follow-up to the correlative study, the four sandstone units and the volumes of effective porosity, effective water saturation, and hydrocarbon pore volume have been entered into Dynamic Graphics Inc.’s earthVision software. The earthVision software provides 3-D visualization of the sand units and the tools for determining volumetrics used in the calculation of reserves.