Two-Dimensional Burial History Model and Geochemistry Shed Light on Petroleum Systems and Mixed Oil in the Vallecitos Area, San Joaquin Basin, California

The Vallecitos Syncline is a westerly structural extension of the San Joaquin basin. Dispersed oil accumulations in the Vallecitos area make oil and gas exploration challenging. Our earlier 1D model indicated that there could be two active source rocks in the syncline: Eocene Kreyenhagen Formation and Cretaceous Moreno Formation.

Biomarker analysis was conducted on 15 oil samples from the syncline. Source-related biomarkers show two genetic groups, which may originate from two different source rocks. Diamondoid analyses for those samples indicate mixtures of oil-window maturity and high maturity oils. A deep, high-maturity source was strongly suggested based on the geochemical features of the samples.

A 2D line along a published cross-section through the deepest part of the syncline was selected to conduct thermal history, basin evolution, and migration analyses. Stratigraphic evidence and modeling suggest that several recent episodes of erosion are required due to folding that removed significant overburden. Thick (~ 2km) overburden rock in the syncline pushed shallow Eocene Kreyenhagen source rock into the oil window around 14 Ma. In contrast, the Cretaceous Moreno source rock reached extremely high maturity (dry gas window) at same time.

Results suggest that in the Vallecitos Syncline the bottom and the top of the Cretaceous Moreno Formation reached thermal maturity at 37 Ma and 18 Ma, respectively. The synclinal Eocene Kreyenhagen Formation became thermally mature at 14 Ma. The 2D model results indicate that the Kreyenhagen Formation has a maximum transformation ratio (TR) of 50% at its base, whereas the Moreno Formation has TR~100%. These results are supported by biomarker and diamondoid geochemistry, which indicate that the Kreyenhagen oils contain a high-maturity component that could originate from the Moreno Formation. The results are consistent with our earlier 1D burial history model in the Vallecitos Syncline. Compound-specific isotope analysis (CSIA) and quantitative extended diamondoid analysis (QEDA) were employed to confirm correlations and determine oil mixtures.

Migration analysis on our 2D profile indicates hydrocarbon loss on both flanks of the cross-section. Effective traps are absent in the cross-section and most of generated hydrocarbons 3probably migrated out of model along strike or perpendicular to it. A future 3D model could better explain the main migration pathways, if additional structural data become available.  

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California