Potential CO2 Sequestration in Oil Field Reservoirs: Baseline Mineralogy and Natural Diagenesis, Kern County, California

Abstract:

Depleted oil fields have been suggested as potential sites for sequestration of CO₂ generated from the burning of hydrocarbons. However, to be effective for removing CO₂ from the atmosphere, the injected CO₂ must remain within the reservoir. The role of atmospheric CO₂ in rock weathering is well known and a growing body of experimental work indicates that under reservoir conditions supercritical CO₂ also reacts with sedimentary rocks. In order to predict the behavior of injected CO₂ in a given reservoir, detailed knowledge of the mineralogy is required. In addition, post-injection monitoring may include analyzing core samples to examine interactions between reservoir rocks and the CO₂. Thus, documentation of the natural diagenetic processes within the reservoir is necessary so that changes caused by reactions with CO₂ can be recognized.

Kern County, California has been a major petroleum producing area for over a century and has three oil fields that have been identified as potential sites for CO₂ sequestration. Two of these, Rio Bravo-Greeley and McKittrick, have no previously published mineralogic studies. Samples from these (and nearby Wasco) oil fields were studied using transmitted-light petrography and scanning electron microscopy. At Rio Bravo-Greeley-Wasco, Kreyenhagen (Eocene) and Vedder (Oligocene) sandstones are mainly arkosic arenites with only small amounts of volcanic rock fragments. Detrital feldspars exhibit wide compositional ranges (up to Or₇₅Ab₂₅ & Ab₅₀An₅₀). Diagenesis has greatly altered the rocks. There are significant amounts of relatively pure authigenic K-feldspar and albite. Small amounts of authigenic quartz, calcite, dolomite, ankerite, kaolinite, illite/smectite, chlorite, zeolite, and pyrite are present. Plagioclase has been preferentially dissolved, with andesine more susceptible than oligoclase. Al³⁺ has been exported from the sandstones. At McKittrick, Temblor sandstones (Oligocene-Miocene) contain up to 33% volcanic rock fragments. Detrital plagioclase is generally less calcic (<Ab₇₀An₃₀) than at Rio Bravo-Greeley. In spite of these differences, diagenesis was similar in both areas. Future modeling must take into account the wide compositional ranges of components; monitoring will need to consider the significant natural diagenetic alteration.