CEMENTATION AND GRAIN REPLACEMENT IN AN ORGANIC-MATTER-RICH MUDROCK: EXAMPLES FROM THE MISSISSIPPIAN BARNETT SHALE, FORT WORTH BASIN, TEXAS

The Mississippian Barnett Shale of the Fort Worth Basin of north-central Texas is an organic-rich, generally siliceous mudrock and a notable hydrocarbon source rock. A suite of examined samples from subsurface core spans thermal maturities from 0.5% to ~2.0% vitrinite reflectance. Compaction and a potentially significant amount of cementation have occurred to transform these samples from high initial porosities at deposition to much lower porosities at present. Widespread grain replacement has also altered the makeup of these rocks.

Although cementation in the submicrometer interparticle pore spaces present in mudrocks can be difficult to image, cementation in larger open spaces such as fractures and intraparticle pores can serve as partial proxies. For example, a fracture was observed that was filled with varying amounts of calcite, pyrite, quartz, albite, barite, and dolomite. Euhedral quartz cement overgrowths have been observed growing into dissolved grain spaces (intraparticle pores) even in the lowest thermal maturity samples. Much of the cementation has to take place relatively early so as to predate migration of bitumen, a process that fills much of the interparticle porosity.

Abundant interparticle calcite cement is seen in concretions and calcite-rich layers and probably formed early in the diagenetic history. Calcified and partly silicified radiolarian tests have been observed in these lithologies.

Where authigenic quartz cement is observed in interparticle pore space it is typically submicrometer in size and commonly requires high magnification cathodoluminescence imaging to resolve from the host detrital quartz. Care must be taken; however, to distinguish common recycled quartz overgrowths from those formed in place. One locale for relatively abundant quartz cement is around the quartz grains that made up tests of disaggregated agglutinated forams. Significant growth of quartz is also associated with recrystallization of siliceous microfossils.

Replacement of detrital feldspar grains by albite is common and is observed in even the lowest thermal maturity samples. The degree to which this grain replacement may also serve as a cement is not known.