Correlating Oil in Austin Chalk Reservoirs with Turonian-Cenomanian Source-Rock Facies Using Biomarkers, Organic Sulfur Compounds, and Hydrous Pyrolysis
Oils of the Pearsall and Giddings fields in Austin Chalk reservoirs are considered to be derived from Turonian-Cenomanian-age source rocks in the Eagle Ford Group and Boquillas Formation. Reservoir oils in the Austin Chalk Formation of Giddings Field, northeast of San Marcos Arch, may be sourced from the Eagle Ford Group, and those in the large Pearsall Field, southwest of San Marcos Arch may represent a facies change within the Boquillas Formation. Pearsall Field oils in the 30 to 40 API gravity range have lower pristane/phytane ratios, higher C35/C34 terpane ratios, and moderately higher sulfur contents than Giddings Field oils in the same API gravity range. However, oils from both fields contain similar extended tricyclic terpane distributions and stable carbon isotope signatures characteristic of Turonian-Cenomanian source rocks of the Gulf Coast. The interpretive issue is whether the oils from these two large fields are from different organic facies in the Boquillas Formation and Eagle Ford Group or are they the result of mixing with high-sulfur oils generated by source rocks in the deeper Smackover Formation.
The Boquillas and Eagle Ford formations have total organic carbon contents (TOC) ranging from 0.7-8.1%. Both formations contain Type II kerogen, but have different organic sulfur concentrations. Hydrous pyrolysis experiments were used to produce crude oil from the Eagle Ford/Boquillas Formation at 310, 330, 355°C for 72 h. Bitumen extracts of the pyrolyzed source rock and their expelled oils were analyzed for specific biomarkers including thiophenes. API gravities of the expelled oils were also similar and ranged from 23 to 44 API, but the samples from Boquillas Formation produced tar plugs during experiments at 310°C and 330°C. C27-diasteranes/C27-steranes ratio values in clastic-rich samples ranged from 0.11-0.19, and in clay-rich and carbonate-rich lithologies ranged from 0.02-0.09. Boquillas lithologies also showed a greater C35/C34-hopane ratios compared to samples from the Eagle Ford.
Dibenzothiophenes showed a greater relative abundance in Boquillas lithologies than in Eagle Ford lithologies. However, the ratios of n-methyl-dibenzothiophenes/total dibenzothiophenes in both lithologies were similar. Preliminary biomarker data indicate facies changes within the formations is responsible for oil-compositional differences in the two fields.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California