Diagenesis of US Gulf Coast shales.

The burial diagenesis of mudstones is of major interest in petroleum geology. Key questions which remain incompletely answered include the relative importance of temperature, time, changes in pore-water chemistry and organic maturation as driving mechanisms, and the relationship between diagenesis in mudstones and neighbouring sandstones. We reconsider here the nature and timing of diagenesis in Gulf Coast Tertiary mudstones using data obtained by backscattered electron microscopy (BSEM) and energy-dispersive X-ray analysis (EDS) of cuttings from two Texas wells. Our observations confirm the mineralogical trends with depth reported by Hower et al. and suggest new evidence concerning the timing and causes of the changes. The most important mineralogical changes occurred in the zone of organic matter decarboxylation (zone IV). We show that the sequence of burial diagenetic events can be established quickly and reliably by BSEM examination of shale cuttings, and demonstrate the use of foram tests and their authigenic mineral infillings as indicators of diagenesis in soft, fine-grained shales.