Depositional environments and sequence stratigraphy of carbonate mudrocks using conventional geologic observations, multiscale electrofacies visualization, and geochemical analysis: The case of the Tuwaiq Mountain and Hanifa Formations in a basinal setting, Saudi Arabia

Abstract

ABSTRACT Depositional interpretation and sequence stratigraphic analysis of carbonate mudrocks requires numerical analysis and data integration to achieve quantitative, predictive stratigraphic and geochemical models. A depositional and sequence stratigraphic analysis is built for a basinal interval of the Tuwaiq Mountain and Hanifa Formations, Saudi Arabia. Conventional geologic interpretation, automated electrofacies analysis, and geochemical interpretation are integrated using quantitative means. Cluster analysis of well logs using self-organizing maps and hierarchical clustering allowed for a multiscale electrofacies analysis. This is useful for identifying major lithological surfaces, which commonly correspond to sequence stratigraphic surfaces. Geochemical data were used for depositional environment interpretation, such as sediment provenance, redox, and paleoproductivity conditions. Factor analysis is used to group element data. Redox and paleoproductivity indices were calculated using electrofacies clustering of different elemental groups. Electrofacies analysis shows good correlation with the lithofacies; that is, lithofacies can be identified from logs. Five major lithofacies have been identified in the studied interval: (1) wispy laminated skeletal wackestones to mudstones; (2) differentially cemented skeletal wackestones, packstones, and grainstones; (3) laminated peloidal mudstones to wackestones; (4) bioturbated packstones and wackestones; and (5) palmate anhydrite. The majority of the interval is interpreted to be deposited by gravity processes. Four major sequences have been identified in the Tuwaiq Mountain and Hanifa Formations made of transgressive and highstand systems tracts. The uppermost bioturbated packstones of the Hanifa Formation are interpreted to be a lowstand systems tract with subsequent restriction leading to the deposition of gypsum. In the studied interval, total organic carbon (TOC) content correlates well with suboxic to anoxic intervals that have high paleoproductivity. Complete anoxia is not a prerequisite for organic matter preservation. These high TOC intervals are mainly in transgressive systems tracts.