9526951 Garven Regional overpressuring is a phenomenon characteristic of the Sacramento Valley and the Coast Ranges of California. The creation and maintenance of high fluid pressures in any sedimentary sequence require two basic conditions: (1) low-permeability confining units and (2) geologic forces which increase the pore pressure within these rocks. The forces may be tectonic, thermal, or even chemical in nature. The intent of this study is to determine which factors play a role in the creation of overpressure zones in the Central Valley of California and to quantify the contribution of each in order to successfully model the high pressures currently found in the region. Sedimentary basins in the Great or Central Valley of California host regional aquifers and at greater depths host large petroleum reservoirs. Geological and geochemical data are abundant, as the Valley has been extensively explored and mapped for these natural resources. The Sacramento Basin in the north is rich in natural gas and geothermal resources, and deep groundwater (depths greater than 700 m) is noticeably overpressured in the western part of the basin near the Coast Ranges. Despite the long history of geologic exploration, very little hydrogeologic work has been done to quantify the nature of deep groundwater flow of geologic processes such as overpressuring in the Great Valley. This proposal seeks to increase our basic understanding of how the deep groundwater flow systems have evolved over geologic time in the Sacramento Basin. Earlier studies on the Sacramento Basin suggest that the origin of anomalously high fluid pressures is related to tectonic compression. According to the conceptual model of Berry (1973), east-west compression associated with the San Andreas Fault and the Basin and Range province "squeezed" the Great Valley sediments like a tectonic vise, driving deep pore waters upward. These forces generated high pressure zones due to the low permeability of the thick marine mudstone sectio n. However, other mechanisms for overpressuring may be important. Petroleum generation, dehydration reactions, crustal heating, and rapid subsidence are other mechanisms that need to be considered and quantified hydrodynamically. A primary objective of this proposal therefore is to use hydrogeologic modeling as a tool for quantifying the relationship between the deep ad shallow flow systems in the Valley and test the viability of conceptual models for overprssuring . Various types of field observations will be used to place constraints on the hydrogeologic models, but mostly we will compile pressure data from deep exploration wells. The work proposed here represents the first effort to quantify "deep" groundwater flow dynamics in the Great Valley of California. Hydrologic testing of geologic concepts with coupled mathematical models form the heart of the study: hydromechanical model of the Sacramento Basin ought to shed basic light on other causes for for the origin of overpressures I the Earth's crust. Furthermore, water resources are precious in the Valley which contains one of the largest fresh-water aquifer systems in the world. Improving our understanding of the basin groundwater systems may have significant practical benefit with regard to understanding of the potential salinity contamination of the shallow, fresh-water aquifers as development continues to satisfy an over increasing demand for potable water.
