A Stratigraphic Model for Hydrocarbon Seep Deposits

Studies of modern and ancient hydrocarbon seep deposits are integral precursors to ultimately understand methane-related reserves. Previous studies of methane seep zones and the limestone byproduct of microbial oxidation associated with serpentinite diapirs show that seep zone fauna and sediment types tend to be distributed randomly. Detailed mapping of the geology of the Wilbur Springs serpentinite diapir (Lower Cretaceous, Colusa County, California) demonstrates a more stratigraphic distribution to limestone facies. Seep-zone facies include (in order of dominance) cement-rich mudstone, brachiopod-rich fossiliferous limestone, coquina, mudstone, limestone breccia, and concretions. Host rock is dominantly serpentinite with minor gabbro and metaconglomerate and felspathic sandstone.

Detailed location and identification of approximately 60 outcrop and subcrop exposures were measured with a sub-meter GPS and the data were downloaded into a GIS. The unexpected results showed that within the serpentinite, the limestone facies occur in a repeating stratigraphic sequence. The sequence begins with mudstone, followed by fossiliferous limestone, with increasing cement toward the top. Concretions occur on the lateral margins of the limestone. These sequences correlate throughout the mapping area and strike in a northeast to southwest direction.

Published geologic maps of Wilbur Springs depict the faulted limestone outcrop within a serpentinite diapir. Comparing the limestone exposure to the current geologic map reveals anomalies in the lithologic distribution. The limestone is not as widespread as shown on the map though the serpentinite contains small, meter-scale pockets of cement-rich and rare fossiliferous limestone.

A closer look at the distribution of ancient hydrocarbon seep deposit byproducts is needed to correctly interpret this important and fragile environment.