Sedimentary architecture and palaeogeography of Lower Slochteren aeolian cycles from the Rotliegend desert-lake margin (Permian), the Markham area, Southern North Sea

Abstract

The Rotliegend gas play in the Southern Permian Basin has yielded over 200 gas fields in the Netherlands; they are found in an E-W fairway along the southern flank of the basin. Sandstones generally pinch out basinward, but localized, isolated sands are present north of the main fairway. The Rotliegend of the Markham gas field and a number of smaller fields in its vicinity (Markham area) provides a good example of such an isolated sand occurrence, and it may serve as a model for exploration in the feather edge of the Rotliegend desert lake. The reservoir interval (Lower Slochteren Member) is a diachronous sequence, 20-50 m thick, from aeolian-dune sandstones to desert-lake mudstones. Periodic fluctuations of lake level, probably controlled by short-period Milankovitch rhythms (precession or obliquity) resulted in the formation of desert-lake mudstone drapes that compartmentalize the reservoir over kilometers. The Lower Slochteren interval consists of four aeolian cycles, 5-15 m thick, which are retrogradational from sharpbased aeolian sandstone, via sandflat and mudflat deposits to desert-lake mudstone. Toward the south the clay-bearing facies pinch out and aeolian sandstones merge into a compound aeolian sandstone body 20 m thick. The aeolian cycles accumulated in an eastward-dipping, 10-km-wide palaeovalley in the Base Permian Unconformity. The cycles onlapped onto the valley margins until the entire valley was filled and a depositional plain came into place. The plain was flooded by the Rotliegend desert lake, followed by the formation of progradational cycles about 5 m thick, each consisting of a basal desert-lake mudstone grading upward into mudflat and sandflat deposits. The change from retrogradational (fining-upward) cycles to progradational (coarsening-upward) cycles seems controlled by the rate of formation of accommodation space during lake-level rise. Initially palaeotopography restricted the creation of accommodation space, thus allowing sediment supply to keep up with rising lake level and forcing dune sands to stack up against rising palaeogeography, resulting in aggradational to retrogradational sequences. However, lake-level rise across the depositional plain caused regional flooding and rapid and far retreat of the lake-margin depositional system, causing accommodation space to be filled after the flooding and resulting in progradational sequences. The Markham case shows that the presence of isolated Rotliegend sandstones is related to palaeotopography and that their internal architecture is controlled by periodic expansion and contraction of the desert lake. It emphasizes the importance of accurate seismic definition of the Base Permian Unconformity and detailed, sedimentology-driven correlation for future exploration at the fringes of the Rotliegend-play fairway. Copyright © 2011 SEPM (Society for Sedimentary Geology).