Petroleum system evolution, strike-slip tectonism, and diagenesis of the Monterey formation in the Salinas Basin, California [electronic resource]
Keywords: Geological and Environmental Sciences.

Since the late nineteenth century, central and southern California Neogene basins have been recognized as prolific petroleum producers. One of these, the Salinas Basin, features at least seven oil fields containing over half a billion barrels of recoverable oil. The mostly uplifted western side of the basin has invited decades of geologic research relating to tectonic evolution of the western California margin, whereas economic interests have focused study mainly on the subsurface features in the east. Though we possess a basic knowledge of the relations between the geology and existence of petroleum in Salinas Basin, we can achieve a more complete understanding by integrating the geological nuances characterizing basin history with petroleum system analysis. The most notable of these traits include strike-slip tectonism and diagenesis of the Miocene Monterey Formation. Late Neogene tectonism associated with the San Andreas Fault led to profound changes to the entire western margin of North America. In the Salinas Basin, this is manifested as numerous, dextral strike-slip and oblique slip faults, one of which includes the Reliz-Rinconada Fault (RRF), which bisects the main depocenter. Transpressional tectonism induced shortening, taken up by broad and small-scale folds and thrust faults. The related partial basin inversion resulted in considerable erosion, including Oligocene and Miocene age strata. In addition to the dynamic tectonism, mechanical and thermal properties of a key stratigraphic formation, the Monterey Formation, evolved with burial and diagenesis of biogenic silica. From diagenetic alteration to tectonic thickening of overburden, to kilometer-scale transport of a pod of active source rock (POASR), all of these processes are intertwined with synchronous petroleum system events. Past research in basin evolution has done little to combine complex tectonism and diagenesis with petroleum systems analysis. Basin and petroleum system modeling (BPSM) is an effective approach to linking these geologic processes and integrating them into a common framework. The Salinas Basin is an excellent area to test and develop BPSM techniques that address the geological complexities often oversimplified or overlooked in standard workflows. This thesis is divided into three chapters addressing the following overarching research objectives: (1) to address the coupling between tectonism and petroleum system formation. (2) To incorporate silica diagenesis to improve geohistory modeling. (3) To reevaluate the Salinas Basin petroleum system(s) in an effort to help explain the oil field size distribution. (4) To integrate historic geologic and petroleum system data and concepts into a digital basin-scale framework. Chapter 1 explores the interplay between silica diagenesis and basin and petroleum system analysis. X-ray diffraction (XRD) analysis of porcelaneous outcrop samples determined that most surface exposures of the Hames Mbr of the Monterey Fm are opal-CT phase silica, with few opal-A phase diatomite occurrences rimming basin margins in the central and southern parts of the basin. X-ray fluorescence (XRF) analysis of the same samples reveal that relative clay abundances for sampled outcrops near the Arroyo Seco Trough (AST) and Hames Valley Trough (HVT) POASRs average 26 wt %. Detailed analysis of samples from a continuous section of Hames Mbr strata in Reliz Canyon (adjacent to the AST) reveals a systematic increase in stability of opal-CT silica with stratigraphic depth. The classic nomograph of Keller and Isaacs (1985) relating diagenetic silica crystallography and clay content to temperature suggests that maximum burial temperatures seen by this section range from 56.4-77.5 °C at 30% clay. One-dimensional (1D) and two-dimensional (2D) burial history models incorporate a dynamic lithology for the Hames Mbr, capable of adjusting properties upon reaching designated temperature thresholds. Observed silica phases coupled with other forms of burial and thermal calibration (porosity, temperature, Tmax, vitrinite reflectance) guide erosion estimates. 1D models of the Reliz Canyon outcrop indicate a broader maximum temperature range and steeper gradient than indicated from crystallographic evidence, suggesting structural tilting of strata prior to uplift. Based on systematic erosion and structural thickening scenario testing of nine wells in the AST and HVT, and corroboration of their burial history using silica phase among other calibration sources, the magnitude of Pliocene-Recent erosion in the AST area increases to the northwest, and ranges from 700-1400 m. 1D and 2D basin models indicate that the eastern part of the basin experienced considerably less erosion (typically 0-80 m) than in the west. Collectively, these models show that locally and at basin scale, erosion magnitude generally increases from south to north, and from east to west across the RRF. The spatial variability in burial and uplift history resulted in diachronous source rock maturation in HVT versus AST: source rock reached 50% transformation ratio (TR; type II kinetics) by late Miocene time in the HVT, whereas AST source rock was approaching 40% TR at present-day. Elements of this chapter will be incorporated into publication of Chapter 3. Co-authors will include Dr. Stephan Graham, and those co-authors mentioned for Chapter 3. My contributions to this work include project design, sample collection, laboratory analysis, results interpretation, collection of data required as modeling parameters and boundary conditions, all model construction and interpretation. Dr. Graham helped with initial project design and overall interpretations. Chapter 2 presents a new BPSM method that incorporates strike-slip fault motion in 3D basin models. No published literature documents the incorporation of strike-slip faulting into basin and petroleum system modeling prior to this work, despite the abundance of petroliferous basins influenced by strike-slip faulting. 3D synthetic models of a simplified version of Salinas Basin demonstrate the feasibility and value of including strike-slip motion in petroleum system analysis. Model results exemplify potential implications for inclusion of strike-slip fault motion in basin models, such as formation of alternative migration pathways through time and mixing of petroleum from multiple sources in the same accumulation. This chapter was submitted to Marine and Petroleum Geology in spring, 2014, and is in review at this time. Co-authors include Dr. Oliver Schenk, Dr. Carolyn Lampe, Dr. Thomas Fuchs and Dr. Stephan A. Graham. My role in this research was in project design, general workflow development, all model construction and testing, and in interpretation of model results. Dr. Schenk and Dr. Lampe provided critical guidance in developing specific aspects of the modeling approach, and provided clarification on technical details. Dr. Fuchs clarified technical details related to the modeling simulator. Dr. Graham helped with initial project design. Chapter 3 investigates the interplay of strike-slip tectonism, basin history and petroleum system development. 3D modeling of basin and petroleum system evolution predicts up to 75 % transformation in the lower Monterey Formation in the main POASR (HVT) and up to 35 % transformation in the western POASR (AST) at present-day. Models predict the largest petroleum accumulations directly northeast of the HVT pod of active source rock, consistent with the location and size of the San Ardo oil field. Very little oil and gas migrated from west to east across the RRF; the little petroleum that is transmitted across the fault accumulated in reservoir layers and fine-grained layers in the northeastern area of the basin. Petroleum generation began (10% TR) at ~11 Ma in the HVT POASR, and accumulation in sandstone reservoirs began ~5 Ma. Accumulation history of the San Ardo area in detail indicates sequential filling of near-source traps in reservoir layers first, followed by accumulation of neighboring traps updip in the last million years. Interpretations of seismic reflection data of the HVT suggest four main tectonic stages. These stages include: (I) Oligocene-early Miocene time is characterized by a transtensional regional stress regime associated with passage of the Mendocino Triple Junction. Evidence for this includes normal faults that progressively down-step basement to the west and north, initiating basin subsidence. (II) Early-middle Miocene rapid subsidence occurred through displacement on high-angle basement normal faults. I suggest this period of subsidence is driven by local transtensional deformation due to a right step-over between subparallel strike-slip faults, resulting in formation of a pull-apart basin. (III) Middle Miocene-early Pliocene time is characterized by a shift in deformational styles, marked by contractile features including low-angle detachment faults within the Monterey Fm, and modest uplift of basin fill in response to this shortening. (IV) Pliocene-Recent shortening associated with the Coast Range Orogeny is evident in the three or more NE-SW striking thrust faults uplifting strata in the west and north. Review of petroleum system models in the context of these seismic interpretations suggests a number of feedbacks between tectonics and petroleum-related events. Biomarker concentrations in sampled oils including C29 sterane and C31 terpane isomers and Ts/Tm ratios indicate relatively low levels of thermal maturity for all oil field samples. An oil produced from much greater depths (~2500 m TVD) appears considerably more mature and less biodegraded than all other samples. Additionally, C27-C29 steranes and monoaromatic steroids suggest a single anoxic marine source with probable subtle variability in organofacies. An abbreviated version of this chapter is planned for future publication with co-authors Dr. J. M ... .