Chapter 43 The Application of Bulk Rock Geochemistry to Reveal Heavy Mineral Sorting and Flow Units in Thick, Massive Gravity Flow Deposits, Siri Canyon Palaeocene Sandstones, Danish North Sea

Abstract

The Palaeocene reservoirs in the Siri Canyon consist of laterally limited gravity flow deposits of glauconite-rich quartzose sand. The sands, which occasionally form up to 40 m thick massive units, are interbedded with deep marine muds, and although they are mainly in-situ deposits, post depositional remobilisation and/or injection have affected certain intervals. This contribution presents a geochemical study of the Siri Canyon Palaeocene sandstones based mainly on whole rock X-ray fluorescence analysis, complemented by thin section petrography. Data presented here is from the Cecilie Field, generally focusing on the type well, Cecilie-1A. Massive sand units of the penetrated succession cannot be subdivided by means of macroscopic core description, overall grain size data, glauconite content or biostratigraphy. However, they can be subdivided into a series of flow units by interpreting the trends of Zr, Th and TiO 2 on the geochemical logs and heavy mineral grain size sorting. In the Cecilie-1A well, four distinct cycles have been recognised in a 41 m section of homogenous, massive sand on the basis of elemental trends, gamma ray and grain density logs, and in factor score depth plots, calculated by Principal Component Analysis (PCA). These trends are interpreted as reflecting systematic heavy mineral variations. A strong correlation is observed between zircon percentages from thin section point counting data and the Zr log, calculated as percentages of whole rock volume. Within one cycle, the upwards decrease in zircon volume percentage within the unit reflects a decrease in zircon grain size and frequency and results from detrital zircon grain sorting through suspension fall-out; this conclusion is supported by PCA. We argue, therefore, that each well-defined cycle represents a single surge within a larger flow event, and that the sediments were sorted by suspension fallout during deposition. Our study demonstrates that heavy mineral grain size sorting may be reliably inferred from petrophysical log patterns. Chemical and petrophysical cyclicity is observed in several wells in the Siri Canyon area, suggesting that suspension fallout sorting of heavy mineral grains may be a common feature in concentrated density flow deposits, allowing the recognition of individual flow units within thick massive sands.