Complex Interplay Between Depositional and Petrophysical Environments on a Holocene Tidal Flat (Al Ruwais, Qatar)
Carbonate rocks can be classified in terms of those properties relating to the pore system of lithified sediments, so called ‘petrophysical rock types’ (PRT), or by ‘depositional rock types’ (DRT), which describe the characteristics affected by depositional environment. Whereas PRTs are typically used to identify and distribute rock bodies within a reservoir with similar flow characteristics, DRTs ignore pore types and capture sedimentary lithology, texture, sedimentary structure and fossils. Both classification systems are extensively used to describe reservoir rocks, however, but the degree of plurality between them remains poorly understood and is the motivation for this study. A field assessment was conducted for a 175 sq. km area situated offshore Al Ruwais, Qatar, which encompasses depositional environments spanning supratidal, intertidal and open marine conditions. A total of 350 surficial sediment samples were collected along 24 shore-normal transects. Each sample was assigned a PRT class based on analysis of grain size and sorting. DRT classes, by contrast, were defined with reference to faunal content and, in turn, classes of mineralogy were delimited by weighting this content against the mineralogy of each faunal category. Next, the discrete samples were interpolated into continuous surfaces describing the distribution of DRTs, PRTs and mineralogy, and spatial correspondence between those surfaces statistically evaluated. In order to link these parameters with environment of deposition, their correlation with water depth (audited from LiDAR) and ecological habitat (mapped from satellite) was also assessed. The data reveal that spatial distributions of sedimentary faunal, petrographic and mineralogical properties do not show exactly congruent patterns. Meaningful trends do exist, however. For example, the occurrence of certain DRTs are indicative of particular PRTs, and vice versa. Connections between PRTs and mineralogy are also emphasized and offer insight as to how the evolution of matrix porosity might be predicted via diagenetic models tuned to specific sediment textures. Useful relationships are also identified between the occurrence of PRTs and DRTs, and both ecological habitat and depth. The potential of such dualities are twofold. First, they can be applied to more realistically distribute PRTs and DRTs by environment of deposition in reservoir models and, second, the use of modern carbonate systems as subsurface analogs might be enhanced.
AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017