Abstract

Recent gas wells have been completed in shallow Precambrian crystalline basement on the Fort Stockton High, Pecos Co., Texas. These rocks occur at depths generally less than 5000 ft and are characterized by both fracture and vuggy porosity. Recent open hole completions have resulted in high initial flow rates with steep declines typical of fracture dominated reservoirs. Although marginally economic, production from these reservoirs can pay for the cost of a well on a “point-forward” basis because of these high initial flow rates. Reservoir characterization of these wells has improved productivity from the basement reservoirs because better-informed decisions can be made during drilling and completion, and simply by recognizing pay in the wellbore. The success of these gas wells has rejuvinated interest in the Precambrian as a target in this mature area.

This study documents how basement composition, fracture systems, weathering, and diagenesis interact to create different pore systems. Major basement rock types have been mapped using cuttings, spectral gamma ray, and density-neutron logs. Petrologic study of basement cuttings from four wells reveals that diorite and granite are the most common rock types in the study area. Diorite cuttings contain dead oil and pyrite in a finely-intercrystalline pore system associated with diagenetic dolomite and minor albite. Vuggy pores, lined with hematite, occur in some diorite/mafic basement samples. Hematite lining pores is interpreted to be the result of weathering. Coarse-crystalline granite and granodiorite exhibit no intercrystalline or diagenetic porosity, although fracture porosity formerly inferred from wireline logs has been confirmed by Formation Micro Imaging (FMI) logs. FMI logs indicate exfoliation fractures predominate with local occurrences of tectonic fracture sets. Intersection of these fracture sets enhances permeability in the basement reservoir. Complex textural and mineralogical zones associated with major fractures or faults have also been observed. In one well a highly altered zone containing abundant chlorite, hematite, limonite and serpentine, and a high thorium response on logs was encountered 120 feet into mafic basement. Subsequent tests and production logs showed this zone to be highly permeable and gas productive.