Managing clastic reservoir heterogeneity I: Sedimentology and sequence stratigraphy of the Late Triassic Minjur Sandstone at the Khashm al Khalta type locality, Central Saudi Arabia

Abstract

The Late Triassic (Norian–Rhaetian) Minjur Sandstone provides a remarkable case study for understanding and modelling the spatial distribution of sand bodies in a fluvial-deltaic system. As such it has been studied in connection with the geological storage of CO2 in complex heterogeneous formations. Detailed sedimentological mapping of the formation’s vertical and lateral variations in and around the type section at Khashm al Khalta (Khashm al Minjur), which is the area of interpreted maximum inlet/outlet activity, has provided a relatively detailed picture of the sequence stratigraphy. As originally described, shallow-marine flooding with the development of tidal mud flats and carbonate facies occurred near the middle of the formation, splitting it into a lower member dominated by subtidal, brackish and scattered fluvial environments, and an upper member marked by the appearance of meandering point bars capped upward by very proximal deposits forming thick (20 m) coarse-grained sandstone bars that can be followed over several kilometers. The general trend at formation scale is thus upward thickening and coarsening sedimentation related to an increasing clastic influx and the development of fluvial systems, with the fluvial upper member being dominated by amalgamated sand bars. The sequence stratigraphy indicates nine depositional sequences involving four depositional environments: sabkha, tidal, estuarine and fluvial-continental. The lower Minjur is a transgressive tract of four sequences of which Sequence 4 reflects maximum flooding and correlates with maximum flooding surface (MFS) Middle Norian Tr80. Sequence 5 corresponds to a meander system at the base of the upper Minjur, and is followed by sequences 6 to 9 reflecting an increasing clastic influx generating amalgamated coarse-grained bars. The upper Minjur thus represents a highstand systems tract.

The Middle to Late Triassic of Central Saudi Arabia with emphasis on the Jilh Formation. Part II: sequence stratigraphy, depositional and structural history, correlations and paleogeography

“…The base of Sequence 4 truncates Sequence 3 and partially Sequence 2 of the Minjur Sandstone as a north-dipping surface observed in the field and modelled (EathVision ® 3D, Figure 3b) by B. Issautier and Y. M. Le Nindre in 2011[annex work to Issautier, 2011. Issautier et al [2019] thus assigned the MFS Tr80 to the early Middle Norian R. wigginsii beds, widespread on both sides of the Qatar Arch as shown by their figure 16 after Stewart et al [2016]. It matches the Alaunian 1 ages obtained in Part I of our study, on top of the Jilh Formation and at the base of the Minjur Sandstone northwards of 25°30 ′ N, from ammonoids and from the conodonts Ancyrogondolella praeslovakensis, and Epigondolella abneptis in several sections.…”

“…A comparable Rhaetian to Early Jurassic transgression can be identified in Kuwait [Kadar et al, 2015]. Issautier et al [2012aIssautier et al [ , 2019 explain the different extents of the R. wigginsii and R. rhaetica transgressions by modified paleogeographic settings. A flatter landscape with tidal flats during Early-Mid Norian enabled retrogradation of R. wigginsii marine facies to more proximal settings.…”

“…This erosional discontinuity is an obvious candidate for an additional sequence boundary, which might be designated as Tr90 SB. Issautier et al [2019] have demonstrated that there are three maximum flooding intervals (MFI) in the Minjur Sandstone through a southwest-northeast transect from outcrop to the Gulf (location on Figure 1):…”

“…More recent publications provide a detailed description and interpretation of the underlying Khuff Formation [e.g. Vachard et al, 2005 and overlying Minjur Sandstone [Issautier et al, 2012a.…”

“…Applying this knowledge to published sections of the Jilh Formation in the Central Ghawar area and eastwards into the eastern Rub al Khali Bains [Deville de Periere et al, 2022] demonstrates the applicability of the scheme at a regional scale. By including new data on Minjur Sandstone stratigraphy [Issautier et al, 2019], this analysis provides a correlation tool for the entire Middle to Late Triassic stratigraphy, extending from the outcrops to subsurface sections at the platform scale regardless of lateral facies changes.…”