Quantifying Tabularity of Turbidite Beds: Methodology and Applications

Abstract

Turbidite deposits architecture at the lowest hierarchical level is defined by the geometry of the individual beds, which may vary from very lenticular to tabular. Tabular beds can form sheet-like deposits, which have been the subject of much research attention; they are regarded as a primary component of high quality hydrocarbon reservoirs, owing to their excellent lateral continuity and predictable geometry. In addition, deposit tabularity is a key piece of evidence used to infer flow confinement in ancient systems and thus to evaluate the suitability of outcrop datasets as reservoir analogues. However, the measurement of tabularity is rarely attempted and a commonly accepted definition on how to describe it quantitatively is lacking.

In this study, published data from ten well-constrained ancient turbidite systems in outcrop were analysed. A methodology for calculating quantitative measures of tabularity from published log panels and photo panels was devised, based on: a) dividing beds into two groups based on their thickness (0.3-1.5 m and 1.5-5m), b) calculating the percentage of beds that are continuous across a fixed window (500 m) and c) calculating the thinning rate of the continuous beds in the same fixed window. When possible, calculations were made for multiple windows within one system, highlighting changes in tabularity along a proximal to distal or axial to lateral transect. The analysis shows a general increase in tabularity with distality, coupled with a decrease in net-to-gross. A comparison between tabularity and the inferred degree of confinement was also undertaken. Results show that although a relationship between degree of confinement and tabularity emerges (e.g., confined/ponded systems show> 70% bed continuity in contrast to a wide spread for unconfined systems), only some of the calculated metrics allow unconfined and confined systems to be differentiated. In addition, different bed types show different characteristic tabularities (e.g., hybrid even beds thinning rates are up to three times those of classic turbidites).

The described methodology provides an effective tool to compare tabularity within and across systems quantitatively. It is hoped that the quantitative determination of tabularity will become a common workflow when describing ancient turbidite systems. This approach can enhance the value of outcrop data to inform models capturing the architecture of systems that are analogues to subsurface hydrocarbon reservoirs.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018