PENNELL, DAWNE A., and ROBERT A. HORTON, JR, Department of Geology, California State University, Bakersfield, CA

ABSTRACT: Cyclic-Steam-Induced Diagenesis in Potter Sand, Midway-Sunset Field, California

Although steam injection is widely used in hydrocarbon extraction, little is known of the effects of steam on reservoir properties. This study documents mineralogic changes resulting from cyclic steam injection into the Potter sand (upper Miocene) at Midway-Sunset field. Samples were obtained from two cores recovered from wells located 200 m (650 ft) apart. The first well was drilled in 1984 prior to initiation of cyclic steam injection; the second well was drilled in 1991 following cumulative injection of 1,000,000 bbl of steam. Samples were obtained from the highest and lowest temperature zones of the reservoir in the post-steam well and from stratigraphic equivalents in the pre-steam well. Depths ranged from 122 m (401 ft) to 313 m (1026 ft). Samples were examined petrographically stained for feldspars) and with x-ray diffraction.

In unsteamed samples, the Potter consists of fine to coarse-grained lithic arenites and conglomerates. Lithics are mainly granitic and volcanic grains with rare metamorphic and sedimentary grains. Pseudomatrix, formed by crushing of quartz, feldspars, and granitic grains and squashing of volcanic grains, ranges up to 18% but is generally less than 4%.

A number of mineralogic changes were noted in the steamed well. Grain dissolution increased dramatically. Quartz, feldspars, lithics, biotite, and pseudomatrix were all affected, with a resultant shift in QFL ratios toward feldspathic litharenite compositions. Sulfates (natrojarosite and gypsum) were completely dissolved. Intragranular porosity increased from a pre-steam average of 0.4% to an average of 1.1%, mainly due to dissolution of lithics. However, average intergranular porosity decreased from 25.4 to 19.7% for a net decrease in total porosity. Traces of calcite cement and pyrite associated with alteration of biotite were observed in the post-steam well. Alteration of framework grains and pseudomatrix abundance were noticeably higher in the steamed well, as were clay coatings, ridges, and pore-filling cements. The relative percentages of clays, as determined by quantitative x-ray diffraction, increased from unsteamed to steamed samples. Changes in clay minerals included formation of smectite, loss of chlorite, a slight decrease in the percentage of clay-sized mica, a large decrease in kaolinite (9.7 to 2.5%), and changes in the composition of mixed-layer illite/smectite: random ordered illite/smectite disappeared in the steamed zone and the percentage of expandable layers in ordered illite/smectite decreased from 50 to 15-20%.

The data collected during this investigation indicate that significant mineralogic changes have occurred as result of cyclic steam injection. These were accompanied by a drop in thin-section intergranular porosity. We did not evaluate permeability or pore size and geometry, but the decrease in intergranular porosity suggests that permeability may have been affected as well.

AAPG Search and Discovery Article #90992©1993 AAPG Pacific Section Meeting, Long Beach, California, May 5-7, 1993.