HR: 14:55h
AN: H13L-06    [Abstracts]
TI: Deep Water Compositions From the Los Angeles Basin and the Origin of Formation Water Salinity
AU: * Boles, J
EM: boles@geol.ucsb.edu
AF: Department of Geological Sciences, Webb Hall, Santa Barbara, CA 93106 United States
AU: Giles, G
EM: giles@umail.ucsb.edu
AF: Department of Geological Sciences, Webb Hall, Santa Barbara, CA 93106 United States
AU: Lockman, D
EM: dlockman@plainsexp.com
AF: Palins Exploration and Production, 5640 Fairfax Ave., Los Angeles, CA 90056 United States
AB: Deep basin formation waters represent original depositional waters that have been modified by diagenetic processes at elevated temperatures and pressures. In addition, they may be diluted by meteoric incursion from elevated structural blocks along basin flanks. It has long been thought that deep basin formation waters have salinities greater than sea water due to various processes like clay membrane filtration or other types of water-rock interaction. However, our work and similar studies in the San Joaquin basin show that formation waters in deep basins are more likely to become diluted rather than concentrated in the absence of soluble evaporite deposits that might underlie the basin. The idea of increased salinity with depth arose from studies in which the underpinning of the basin consisted of soluble evaporate deposits such as the Texas Gulf Coast, Illinois, Michigan, and some North Sea areas. There are very few deep formation water analyses from the Los Angeles Basin. Furthermore, very few of the current produced waters from any depth can be considered pristine because of the widespread formation water injection programs and commingling of fluids from different levels. Here, we describe the first analyses from a deep, previously untouched part of the basin that is currently being developed in the Inglewood Oil Field. We have analyzed a suite of formation waters from the mid-Miocene marine Sentous sandstone from sub-sea level depths of 2250 m to 2625 m at temperatures of about 110 to 126°C and pressures of about 27 MPa. The original depositional waters in the Sentous Formation were sea water whereas the sampled waters are diluted by about 20% from sea water and some show as much as 50% dilution. Based on comparison of oxygen and deuterium isotopes between the meteoric water trend and these waters, we conclude that the smectite to illite dehydration reaction is the major cause of dilution to the original formation water. Other notable differences include elevated Ca/Na ratios and K/Na ratios which we attribute to dissolution and albitization of feldspar, mainly plagioclase, at the elevated burial temperatures. As is typical of most formation waters, Mg, Fe, and sulfate ions are highly depleted due to early formed pyrite, Fe-Mg siderite, and dolomite.
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1051 Sedimentary geochemistry
DE: 1065 Major and trace element geochemistry
DE: 1719 Hydrology
DE: 3675 Sedimentary petrology
SC: Hydrology [H]
MN: Fall Meeting 2005