--> --> Fractured Precambrian reservoir characteristics: St. Johns CO<sub>2</sub> Field, Apache County, AZ

AAPG Pacific Section and Rocky Mountain Section Joint Meeting

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Fractured Precambrian reservoir characteristics: St. Johns CO2 Field, Apache County, AZ


Granite, granitic gneiss, and mylonite, with varying degrees of fracturing and brecciation, comprise the primary reservoir in St. Johns CO2 Field. U-Pb dating indicates there are two Precambrian units in the field aged 1.6 and 1.4 b.y. Primary storage and production of gas-phase CO2 comes from large high-angle fractures and low-angle joints in the basement. Some of the fractures link to the disconformably overlying Permian granite wash, where CO2 was first discovered. Image logs reveal the fracture and joint system as well as the strongly foliated structural fabric of the mylonitic rocks. To observe the reservoir below the scale of image logs, 52 thin sections of the basement and 1 of the granite wash were examined. Thin section microscopy reveals both ductile (mylonitic) and brittle (cataclastic) deformation fabrics in the St. Johns rocks. Foliation is crosscut by both subparallel and conjugate fracture sets at the megascopic scale. In thin section, fractures were observed partly cemented with kaolinite, dolomite, quartz, or K-feldspar. Mineral and fracture-fabric pores and micropores are also documented in thin section using epifluorescence microscopy (EPI). Microbreccia zones of finely comminuted, microporous material termed gouge or cataclastite surround and grade into many fractures. Intracrystal fractures and microfractures are localized along the cleavage planes in feldspar crystals. Plagioclase dissolution pores are observed in most samples. Well testing and performance histories confirm that the complex fracture network as revealed in logs and thin section is the key to the reservoir's productivity.