An exhumed palaeo‐hydrocarbon migration fairway in a faulted carrier system, Entrada Sandstone of SE Utah, USA

Abstract

The Moab Anticline, east‐central Utah, is an exhumed hydrocarbon palaeo‐reservoir which was supplied by hydrocarbons that migrated from the Moab Fault up‐dip towards the crest of the structure beneath the regional seal of the Tidwell mudstone. Iron oxide reduction in porous, high permeability aeolian sandstones records the secondary migration of hydrocarbons, filling of traps against small sealing faults and spill pathways through the Middle Jurassic Entrada Sandstone. Hydrocarbons entered the Entrada Sandstone carrier system from bends and other leak points on the Moab Fault producing discrete zones of reduction that extend for up to 400 m from these leak points. They then migrated in focused stringers, 2–5 m in height, to produce accumulations on the crest of the anticline. Normal faults on the anticline were transient permeability barriers to hydrocarbon migration producing a series of small compartmentalized accumulations. Exsolution of CO2 as local fault seals were breached resulted in calcite cementation on the up‐dip side of faults. Field observations on the distribution of iron oxide reduction and calcite cements within the anticline indicate that the advancing reduction fronts were affected neither by individual slip bands in damage zones around faults nor by small faults with sand: sand juxtapositions. Faults with larger throws produced either sand: mudstone juxtapositions or sand: sand contacts and fault zones with shale smears. Shale‐smeared fault zones provided seals to the reducing fluid which filled the structural traps to spill points.

Evolution of hydrocarbon migration style in a fractured reservoir deduced from fluid inclusion data, Clair Field, west of Shetland, UK

“…Red sandstones often show extensive bleaching due to the reduction and removal of iron by reducing solutions. This type of bleaching in similar rocks has been attributed to the invasion of hydrocarbons (Surdam et al, 1993), with the areas of bleaching indicating the precise pathways of hydrocarbon migration (Garden et al, 2001). Experimental, empirical and theoretical evidence indicates that other reducing reagents, such as hydrogen sulfide, organic acids and methane can also bleach red sandstones (Parry et al, 2004).…”

Desert Potholes: Ephemeral Aquatic Microsystems

“…Two BARW potholes, 2-10 (B2100202) and 1-06 (B1060401), plot in the lower right-hand quadrant indicating elevated SO 4 values. It is not clear why these potholes have elevated values, but some of this may be a function of proximity to mineralization along the Moab fault (Garden et al, 2001). The position of pothole 99-7 (B9970401) on the plot is characterized by high values of Fe (381.8 lg/L) and Mn (344.4 lg/L).…”

Contrasting red bed diagenesis: the southern and northern margin of the Central European Basin

“…Burley 1984;Kilgore and Elmore 1989;Parry et al 2004). Many authors consider HC as most likely reducing agents (Surdam et al 1993;Garden et al 2001;Parry et al 2004). Other possibilities to remove hematite from red beds include iron reduction by organic acids or hydrogen sulphide, pH decrease by influx of CO 2 , or microbial Fe-removal at temperatures below 120°C (e.g.…”

“…A number of well documented case studies clearly indicate that sandstone diagenesis may be influenced by organic-rich fluids, if HC source rocks are close-by (e.g. Burley 1986;Parnell and Eakin 1987;Muchez et al 1992;Saigal et al 1992;Gaupp et al 1993;Macaulay et al 1998;van Keer et al 1998;Garden et al 2001). Gaupp et al (1993) explains this for the southern part of the CEB, where Rotliegend sandstones are directly in contact with Carboniferous Coal Measures via faults.…”