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AAPG Bulletin

AAPG Bulletin, Preliminary version published online Ahead of Print 10 January 2022.

DOI:10.1306/0104202221069

Different scales of salt-sediment interaction during passive diapirism

Mark G. Rowan1 and Katherine A. Giles2

1 Rowan Consulting, Inc., 850 8th St., Boulder, CO 80302, USA
2 Institute of Tectonic Studies, Dept. of Earth, Environmental, and Resource Sciences, The University of Texas at El Paso, 500 W. University Ave., El Paso, TX 79968, USA

Salt-sediment interaction during passive diapirism typically occurs at two distinct scales that are related to depositional relief from the highs above diapirs to the lows of adjacent depocenters. At the large scale, subsidence into salt generates horizontal to gently dipping depositional surfaces and consequent minibasin tectonostratigraphic successions (MTS). These have widths of multiple kilometers and geometries categorized as: (i) layer MTS, with strata maintaining their thickness approaching the diapir: (ii) thinning-wedge MTS, in which the bounding surfaces converge toward the diapir; and (iii) thickening-wedge MTS, where they expand toward the diapir. Internally, wedge MTS comprise smaller-scale stratal wedges that are concordant, discordant, base-discordant, or top-discordant. Minibasin fill may form a single MTS or have one or more switches between different MTS types. At the small scale, drape-folding of the roof over the steep diapir edge generates a topographic scarp and resultant diapir-flanking, unconformity-bound halokinetic sequences (HS) that are less than 1 km wide. HS stack into composite halokinetic sequences (CHS): hook HS form when the roof is thin and stack into tabular CHS within 200 m of the diapir; and wedge HS represent thicker roofs and stack into broader, tapered CHS. Distinguishing between the two scales of salt-sediment interaction is important because they have different implications for exploration and production in three-way traps against diapirs or equivalent welds. Different stacking patterns of MTS impact risk assessment of trap size, reservoir thickness trends, and hydrocarbon charge, whereas CHS complicate the updip portions of traps and impact both reservoir distribution and seal.

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