ORGANIC-MATTER PORES – NEW FINDINGS FROM MUDROCKS AT LOWER THERMAL MATURITY (RO <0.7%)

New analyses focused on low-thermal-maturity (R_o <0.7%) organic matter (OM) of possible or probable terrestrial origin (Type III kerogen) from three units, contradict previous work on low-thermal-maturity mostly marine OM from the Barnett and Eagle Ford Shales. In early work on mudrocks with primarily marine OM, at R_o <0.75%, OM was observed to typically lack pores, whereas more thermally mature OM tended to develop pores. However, this pattern was not necessarily seen by researchers working on other units, who did note pores in OM at lower thermal maturities. In order to understand if inherited pores in terrestrial OM are responsible for this discrepancy, this study has focused on terrestrial OM at low thermal maturities, which should not have thermally generated pores.

Samples of the Wilcox Group of East Texas (Paleocene) are from two beds of OM-rich terrestrial mudrock. The mudrock has an R_o of ~ 0.5%, a siliceous-argillaceous composition, and >10% TOC. Based on Rock Eval pyrolysis and depositional setting, the OM is thought to be dominantly Type III. Some OM grains in these samples have inherited pores. Pore sizes are up to 5 µm in length and up to 27% of the OM is porous. Pores seem more common in the larger OM grains.

Samples from one core in the basal Eagle Ford Group (Cretaceous) of south Texas have abundant vitrinite (R_o ~ 0.45%), in contrast with shallower layers, which are vitrinite-poor. Rock-Eval pyrolysis also shows possible Type III kerogen. These basal mudrock samples are more argillaceous, but were deposited in a marine environment. Some OM in these samples has inherited pores. Some of these pores are elongate with sizes >1 µm.

Smithwick Shale samples (Pennsylvanian) from the Fort Worth Basin have thermal maturity in the range 0.4 to 0.7% R_o. Although much of the unit is marine, abundant plant fossils suggest a mixed OM source. Some Rock-Eval pyrolysis results also indicate Type III kerogen. Many OM grains show a texture that had not been previously observed. Instead of being a mass of OM with spherical holes, the OM grains are made up of small spherical OM bodies. These OM bodies have variable size, but most are 60 to 100 nm in diameter. Pore space is present between the OM bodies. Several possibilities exist for the formation of these OM bodies, but they strongly resemble so-called “nannobacteria” described by other researchers.