Are Shoal Reservoirs Discrete Bodies? A Coquina Shoal Outcrop Analogue From the Mid Triassic Upper Muschelkalk, Sw Germany

Abstract

Carbonate ramp reservoirs may show a much higher degree of complexity than the schematic and often layer‐cake type depositional models which are widely used in exploration and reservoir characterisation, especially in three dimensions. This complexity was investigated in an outcrop study of the Ladinian (Middle Triassic) Quaderkalk Formation (Upper Muschelkalk) from SW Germany. The formation was deposited on an epicontinental, gently inclined carbonate ramp in the regressive part of a third‐order depositional sequence and represents a sub‐seismic‐scale coquina‐dominated shoal reservoir analogue. In the study area, coquina carbonates make up a complex system of four stratigraphically separated shoal complexes. The present paper investigates the 1‐D to 3‐D facies distribution, reservoir architecture and sequence stratigraphic evolution of the largest shoal complex of the Quaderkalk Formation, the Oberer Hauptquader. The object of this outcrop analogue study was to improve the understanding of internal reservoir heterogeneities (such as connectivity and continuity) in similar carbonate shoal complexes in the subsurface. To that end, detailed analyses of 71 quarry outcrop and core samples were carried out, together with the study of more than 400 thin sections, the 2‐D analysis of three quarry wall panels and regional‐scale 2‐D correlations, and facies mapping. This investigation documents significant sedimentological heterogeneities at different scales within the studied shoal reservoir analogue. Instead of a continuous facies belt, a number of separate potential reservoirs form a shoal complex “mosaic”. Shoal development is controlled by sequence stratigraphic architecture, mainly by cycles which were responsible for shoal expansion during times of regression (forming potential reservoir‐volume) and shoal drowning during times of transgression (forming seals and potential stratigraphic traps). Within the individual shoal complexes, thickness and lithofacies types show gradational lateral changes. Correspondingly, lithofacies associations within the studied shoal analogue are not defined by sharp boundaries. Thus the present “shoals” do not in fact represent discrete “bodies”, as often depicted in reservoir models and as commonly used in reservoir simulation. In addition, shoal‐internal sedimentological heterogeneities (potential flow baffles in the subsurface) are strongly influenced by a combination of small‐scale cyclicity and event‐type deposition. This study therefore demonstrates the need for sedimentological studies of reservoir heterogeneities at various scales in order to decrease uncertainties and risks in exploration and production.

Asymmetry of an epicontinental basin—facies, cycles, tectonics and hydrodynamics: The Triassic Upper Muschelkalk, South Germanic Basin

“…A comprehensive facies atlas applicable for both outcrop and borehole data for the whole of the southern Muschelkalk Basin including its French, Swiss and Luxembourg sections is presented here. It integrates new observations and expands on previously established lithofacies schemes ( Figure 5) for the eastern part of the Muschelkalk Basin in Baden-Württemberg and Bavaria (Braun, 2003;Köhrer et al, 2010;Kostic & Aigner, 2004;Palermo et al, 2010;Petrovic & Aigner, 2017;Ruf & Aigner, 2004;Schauer & Aigner, 1997a, 1997b. These schemes were made for specific ramp sub-environments (such as shoals) but did not cover the entire Upper Muschelkalk facies in the Southern Germanic Basin.…”

“…Gently inclined carbonate ramps developed along the coastal margins of the Vindelician Massif and the London/Brabant Massif directed towards the basin centre. Particle-rich shallow water carbonates were deposited in zones parallel to the coastline (Aigner, 1985;Demonfaucon, 1982) forming shoals often located on top of local palaeohighs like the Siercker High or the Gammesfelder High akin to modern shoals in the Persian Gulf (Braun, 2003;Dittrich, 2011;Duringer & Vecsei, 1998;Gittinger, 1969;Hohage, 1996;Lucius, 1941;Palermo et al, 2010Palermo et al, , 2012Petrovic & Aigner, 2017;Purser, 1973;Rössle, Himmerkus, & Dittrich, 1999;Vecsei, 1998;Wagner, 1989). During the 3rd-order transgression, they retrograde towards the coast.…”

“…Additionally, 18 logged quarries and 15 logged borehole cores generated by previous studies (Braun, 2003;Köhrer et al, 2010;Kostic & Aigner, 2004;Palermo et al, 2010Palermo et al, , 2012Petrovic & Aigner, 2017;Petrovic et al, 2018;Ruf & Aigner, 2004) were revaluated by visiting outcrops and proof-checking existing logs to incorporate them into this study. Field logs were digitized and visualized in WellCad 5.0.…”

“…They are on average 0.3 cm thick, but 1.5 m thick mini-cycles are occasionally observed. Köhrer et al (2010) and Petrovic and Aigner (2017) used mini-cycles for localscale correlations. The identification of mini-cycles may be challenging as they are often poorly preserved, particularly in a foreshoal to offshoal setting, and/or contained in surfaces.…”

“…This study draws on a detailed stratigraphic framework stemming from nearly 200 years of scientific research within the German Muschelkalk Basin (von Alberti, 1834;Aust, 1969;Paul, 1936;Vollrath, 1955a,b,c;Wagner, 1913), three decades of modern sedimentology and sequence stratigraphic studies (Adams & Diamond, 2017;Aigner, 1985;Aigner & Bachmann, 1992;Aigner, Hornung, Junghans, & Pöppelreiter, 1999;Franz, Henniger, & Barnasch, 2013;Franz et al, 2015;Köhrer, Heymann, Prousa, & Aigner, 2010;Kostic & Aigner, 2004;Palermo, Aigner, Nardon, & Blendinger, 2010;Palermo, Aigner, Seyfang, & Nardon, 2012;Petrovic & Aigner, 2017;Petrovic, Aigner, & Pontiggia, 2018;Ruf & Aigner, 2004;Vecsei & Duringer, 2003) and includes new and/or revised outcrop and borehole data (this study). While these previous modern studies provide an extensive and unique database for the Upper Muschelkalk, they only do so for selected palaeogeographic locations and stratigraphic intervals.…”