Pacific Section AAPG, SPE and SEPM Joint Technical Conference

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Integration Of Microstructural And Macrostructural Indicators With Large-Scale Models To More Accurately Measure Shortening Across The Santa Maria Basin, California: Implications For Reservoir Geology And Tectonic History

Abstract

Constructing or testing structural models that predict large amounts of shortening between rotating crustal blocks require an accurate measurement of the total amount of shortening in different locations. It is clear from field examination that a significant amount of tectonic contraction is not included in balanced cross-sections that were constructed assuming volume-conservative structural panels, that are themselves internally deformed. This study will test and refine estimates of tectonic shortening from pre-existing studies across the Santa Maria basin (SMB) and serve as calibration for similar studies of other basins along the California borderland. Additionally, these basins are from great geological and economical importance because they are located at the junction of important tectonic provinces and contain thick Neogene sedimentary sections that comprise high quality source and reservoir rocks of the Monterey Formation. However, an accurate measurement of the total amount and style of shortening across the SMB is necessary to: 1) Test structural models of tectonic rotation in southern California that predict large amounts of shortening across the western SMB due to differential rotation of crustal blocks on either side of the basin. 2) Document heterogeneity of deformation with respect to structural position in order to predict whether brittle Monterey rocks are fractured everywhere or only within/near major structures and evaluating their potential for forming unconventional reservoirs. 3) Understand and characterize tectonically-related loss or creation of porosity (e.g., horizontal compaction of diatomites or dilation brecciation of cherts and porcelanites). Specific tasks that would be completed as part of this study include: 1) Cross-sections at 1 to 5 km scale will be made from geologic maps to calculate strain. 2) Structural mapping and restoration of shortening will be performed at outcrops that are well suited for measuring shortening amounts in the same orientation as the larger cross-sections. 3) Microstructural analyses will be conducted using thin-sections of oriented rock samples to calculate shortening. This task takes advantage of well-documented observations that even in mechanical units that appear to be undeformed, significant amounts of microstructural deformation are observable in thin-sections in some shallowly buried, highly siliceous rocks.