Integrated core, well log, and seismic interpretation of Albian patch reefs in Maverick Basin, SW Texas

The Albian Glen Rose Formation has been well known as a shallow shelf carbonate with common patch reef development along the interface between the Comanche Shelf and the Maverick Basin. Stratigraphic analysis of 75 well logs and 7 seismic stratal slices formed the basic data for interpreting the Lower Glen Rose sequence framework and paleogeographic setting. The Lower Glen Rose units record the HST of sequence 6 and all of sequence 7. Patch reefs were found in the TST, near the MFS, and the HST of sequence 7, with three distinct reef levels found throughout units 2-4. Buildups in unit 2 change from transgressive isolated buildups into a flat biostrome during progressive onlap. Unit 3 buildups record a landward shift relative to the biostrome of unit 2, as shown on well logs and are coincident with the sequence 7 MFS. The third and uppermost buildup level is found in unit 4, in the late highstand of sequence 7. This unit shows a distinct seaward shift of the patch reefs, relative to unit 3, consistent with the highstand interpretation. Detailed examination of facies in the two cores available for study indicates that the patch reefs formed during shoaling of the shelf. These patch reefs are characterized by a low API GR value (<20 GAPInorm) and strong negative seismic amplitudes. Off-reef deposits correlate with a higher, variable gamma-ray response (>20 GAPInorm) and positive seismic amplitudes. The patch reefs are proven hydrocarbon reservoirs with gas potential concentrated in units 2 and 4. Geomorphometric analysis based on seismic stratal slices of more than 30 buildups from unit 4 determined that these are domal features with a distinct NNW-SSE elongation and steeper SW flanks. Asymmetry of the patch reefs may be the result of NW-directed far-field paleo-currents and NNE-directed, wind-driven paleo-waves. Independently of the size scale, the elongation axis of buildups is 1.5-2.5 times larger than its perpendicular direction. Results imply best reservoir facies in the SW side of the Lower Glen Rose patch reefs