Thermohaline and Hydrocarbon Related Diagenesis of Lower Miocene and Upper Oligocene Sediments, West Hackberry Field, Cameron Parish, Louisiana: Implications for Mass Transport and Fluid Flow.

The diagenesis of Lower Miocene and Upper Oligocene sediments flanking the West Hackberry salt dome are documented in order to determine the effect the presence of salt domes may have in driving diagenetic reactions in detrital siliciclastic sediments. These sediments have undergone significant chemical diagenesis during progressive burial resulting in a reduction in porosity and permeability. Precipitation of analcime, siderite, and calcite plus the alteration of detrital volcanic lithic fragments and replacement of aluminosilicate framework grains by calcite are the dominant reactions which have altered the Upper Oligocene sands. Analcime and siderite are also present in the Oligocene mudstones. The isotopic composition of the Oligocene calcites is quite variable suggesting multiple generations of calcite cement, an early one precipitated from seawater and perhaps several later ones precipitated during progressive burial by warm, hydrocarbon enriched, diagenetic pore fluids similar in composition to present day brines. Petrographic evidence suggests and extraformational source is required to account for the occurrence of these authigenic minerals. The most likely sources include the dissolution of halite in the adjacent salt dome, the dissolution and alteration of volcanic lithic fragments and aluminosilicates, and the smectite to illite transformation in more deeply buried sediments. The dominant reactions effecting the composition, porosity, and permeability of the Lower Miocene sands are precipitation of calcite and iron sulfides and replacement of detrital framework grains. Iron sulfide nodules of complex mineralogy and paragenesis occur in interbedded mudstones. The isotopic composition of the calcite is relatively constant and is consistent with the hypothesis that dissolution of Jurassic anhydrite in the adjacent salt dome and oxidation of methane are the primary sources for Ca and CO$\sb3$ respectively. Salt dome anhydrite is also the likely source for the sulfur required for Miocene iron sulfide cements. The diagenesis of Lower Miocene and Upper Oligocene sediments results from the interaction of high NaCl, hydrocarbon enriched brines with the detrital mineral assemblage during progressive burial. These deposits require a dynamic open geochemical system where large quantities of material were transported into the system, most likely due to a dynamic fluid flow regime