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Sur Basin, California

Sur Basin Fig 1 - Transformation Ratio
Sur Basin Fig 2 - Burial History
The Sur Basin (informally called the Partington Basin) is an ~1800 km2, asymmetric, structural basin offshore of the southern part of the central California margin, bounded to the south by the San Martin structural discontinuity, and to the northeast by a nearshore fault. The structural discontinuity separates the undeformed basement of the Sur Basin from the irregular basement to the south. The Sur Basin is the northwestern extension of the offshore Santa Maria Basin, and therefore shares some of its seismic stratigraphy, geologic history and tectonic history. Sulfur-rich type IIS kerogen in the Miocene Monterey Formation source rock in the Santa Maria Basin has generated significant amounts of heavy, sulfur-rich crude oil. The Sur Basin and northern Santa Maria Basin have never been commercially explored.
 
Graduate student Keisha Durant investigates in this study whether the Monterey Formation in the Sur Basin has also generated oil using a 3D PetroMod model. There is no well data or outcrop samples available from the Sur Basin, therefore rock units, the age of unconformities, and the timing of geologic events have been inferred indirectly from evidence in neighboring basins through correlation with seismic data available in the Sur Basin. The stratigraphic input for the model were isopach maps in two-way travel time of the Upper and Lower Foxen, Upper and Lower Sisquoc, and the Monterey formations created in a study by Pankow (1997). The maps were converted to depth using a time-to-depth conversion relationship from a well in the Santa Maria Basin and imported into basin modeling software.
 
In this study, the petroleum system is described using the concepts of Magoon and Dow (1994).  Because the different members of the Monterey have not previously been distinguished in the Sur and northern offshore Santa Maria Basins, for discussion purposes we have designated the lower calcareous-siliceous member, the middle carbonaceous marl member, and the upper clayey-siliceous member as Monterey3, Monterey2, and Monterey1, respectively.  The simulated 3-D model predicts that 28.76 Million barrels (MMbbls) of petroleum has accumulated within the Sur and northern Offshore Santa Maria regions.  No significant gas accumulations were predicted by the model.
 
The Monterey3 source rock layer is more thermally mature than the Monterey2 and Monterey1 members, and therefore most of the accumulations predicted by the model were generated by the Monterey3 member.  At the region of maximum overburden within the Sur area, the Monterey3 begins oil expulsion at burial depths of 2.4 to 3.4 km (@TR=10% : see table 2).  Initial oil expulsion in the Monterey3 occurs between 5.4 to 7.66 Ma with vitrinite reflectance values ranging from .55 to .64% at initial oil expulsion.  The base of the Monterey3 member in the region of maximum overburden within the Sur area has a present-day predicted vitrinite reflectance of 1.15 % and a present-day transformation ratio of  85.57%.
 
At the region of maximum overburden within the northern Offshore Santa Maria area, the Monterey3 begins oil expulsion at burial depths of 3.3 to 4.2 km.  Initial oil expulsion in the Monterey3 within the northern Offshore Santa Maria occurs slightly later than in the Sur area between 4.17 and 5.56 Ma with a vitrinite reflectance of .58% at initial oil expulsion.  The base of the Monterey3 member in the region of maximum overburden within the northern Offshore Santa Maria area has a present-day predicted vitrinite reflectance of .84 % and a present-day transformation ratio of 56.27%.