The Vallecitos Syncline is a westerly structural extension of the western San Joaquin Basin, a prolific oil and gas province in central Californa. Cumulative oil and gas production through 2007 in the Vallecitos area reached 5.4 MMBO and 3.9 BCF, respectively (California Division of Oil, Gas, and Geothermal Resources, 2008). However, dispersed oil accumulations in the Vallecitos area make the oil and gas exploration challenging. To better understand the petroleum systems in the areas and demonstrate local, rather than distant, oil generation, graduate student Meng He has completed 1-D and 2-D models of the Kreyenhangen-Temblor(!) and Moreno-Nortonville(?) petroleum systems in the Vallecitos Syncline. A pseudo-well constructed in the axis of the syncline where the overburden above the Moreno Formation is thickest revealed that both the Moreno and Kreyehagen Formations were thermally mature for petroleum generation. However, with a transformation ratio of only 18%, He concluded that the Kreyenhagen Formation was not the dominant source rock in the syncline, in contrast to the conclusions from previous studies.
A subsequent 2-D model of the Vallecitos Syncline was constructed along a published cross-section where potential source rocks were buried most deeply. This cross-section incorporated significant erosional events on the flanks of the syncline, where inverted and folded rock units imply loss of overburden. Additionally, three heat flow scenarios were tested: 1) a “hot” model resulting from the passage of the Mendocino Triple Junction about 12 Ma; 2) a “cold” model resulting from the refrigeration of subsurface rocks due to the paleo-subducted slab starting about 75 Ma; and 3) a “combo” model incorporating both relatively high and low heat flows. Regardless of heat flow scenario used, model results suggest that in the Vallecitos Syncline the Cretaceous Moreno Formation reached thermal maturity, generating gas in the deepest part of the syncline and oil on its flanks. The synclinal Eocene Kreyenhagen Formation became thermally mature at 14 Ma in the “combo” heat flow model, but remains thermally immature on its flanks. The 2D model results indicate that the Kreyenhagen Formation has a maximum transformation ratio (TR) of 50% at its base, whereas the Moreno Formation is completed transformed at present day.
To complement the basin models, biomarker analysis
was conducted on 26 oil samples from the syncline. Source-related and depositional-related biomarkers show two genetic groups, which may be sourced separately by two different source rocks—presumably the Moreno and Kreyenhagen Formations. Diamondoid analysis of those oil samples indicate mixed oils comprised of normal-maturity (i.e., oil window) components and high maturity components. A deep, high maturity source in this area was strongly suggested based on the geochemical features of the samples. The Moreno Formation could be a deep, previously undetected source rock contributing high-maturity components to most of the mixed oil samples in the Vallecitos Syncline.