Abstract: Subsurface Analysis of Upper Cretaceous Strata of the San Juan Basin; New Mexico: Implications for Basement Structure, Fracture Zones, and Producing Trends

KOHLES, KEVIN M., and ROBERTS, JOHN E.

The San Juan Basin of northwestern New Mexico and adjacent southwestern Colorado contains a thick sequence of Upper Cretaceous strata which is dominated by two major transgressive-regressive cycles. These cycles are represented by the marine Mancos and Lewis shales and their associated coastal sandstones, Dakota, Gallup, Point Lookout, Cliff House, and Pictured Cliffs. The sandstones are typically fine-grained but form extensive gas-bearing stratigraphic traps. Because the sands have relatively low porosity and permeability, fractures are often an important component of reservoir quality. Determining the patterns and trends of the regional fracture system in the San Juan Basin leads to prospective areas for future development.

Fracture trends were delineated by utilizing a comprehensive subsurface data base containing correlated horizons for over 20,000 wells in the basin. Consistently correlated time-stratigraphic horizons, particularly within the marine shale sections, were selected and mapped as structural surfaces. Each of these surfaces were used to generate a trend surface which was subtracted from its structural surface to create trend residual; maps for the selected horizons. These trend residual maps reveals distinct intersecting linear trends, mainly oriented NW - SE and SW - NE, which correspond to very subtle flexures on the structure maps. These linear trends correlate with cumulative production trends and basement features and thus are interpreted as zones of weakness related to basement faults where fracture density is higher.

AAPG Search and Discovery Article #90946©1997 AAPG Rocky Mountain Section Meeting, Denver, Colorado