Recognition of Depositional Environments Using Programmed Pyrolysis: Sparta Formation, South-Central Louisiana

Elizabeth W. Chinn, Rowdy Lemoine, Roger Sassen

Programmed pyrolysis of mudstone provides a rapid and effective technique to recognize environments of deposition in the subsurface. Ninety-three mudstone samples from four downdip Sparta Formation wells in south-central Louisiana were analyzed using a Rock-Eval pyrolysis analyzer with total organic carbon (TOC) module. TOC and the hydrogen index were found to be the most useful parameters for distinguishing depositional environments. Detailed conventional core analysis demonstrates that the Sparta Formation in the study area represents a prograding barrier island complex consisting of reservoir-quality shoreface, storm-washover, and tidal-inlet sandstone. The sandstone overlies open marine mudstone and is capped by lagoonal mudstone. Mudstone of both facies is characteri tically dark brown to black, laminated, and organic rich. However, pyrolysis results reveal geochemical contrasts that can be used to distinguish between the marine mudstone facies and the lagoonal mudstone facies. Lagoonal mudstone has TOC values averaging 1.37% and hydrogen indices averaging 160. In contrast, offshore marine mudstone has much higher TOC values, which average 2.61%, and a dramatically higher average hydrogen index of 341. Average T_max of mudstone from both facies is 431°C, and S₁/S₂ ratios are well below unity, showing that the Sparta Formation is thermally immature in the study area. Thermal maturity differences cannot be invoked to explain the observed geochemical differences between environments. The clear contrast of organic geo hemical parameters is interpreted to reflect dissimilarity in kerogen types and in modes of preservation between mudstone facies. Lagoonal mudstone contains mostly terrestrial kerogen, whereas more oil-prone marine kerogen predominates in marine mudstone.

AAPG Search and Discovery Article #91036©1988 GCAGS and SEPM Gulf Coast Section Meeting; New Orleans, Louisiana, 19-21 October 1988.