Acquisition of Mud by Gravity Flows and the Development of Turbidites vs. Hybrid Event Beds: Insights From the Ponded Castagnola System (NW Italy)

Abstract

Constraining the range of styles in which deep-water gravity flows evolve along their flow pathway is key to developing better understanding of the wide variety of bed types that are observed (e.g., turbidites, debrites and hybrid event beds) and their distribution in deep water systems. Flow evolution can occur due to a number of mechanisms, such as water entrainment, particle fractionation, and sediment deposition or erosion. Hybrid event beds (HEBs) are bi- or tripartite event beds with a lower clean sandstone division and an upper or central chaotic mud-rich division. These beds include features of both turbidites and debrites and their origin has been related to flow evolution due to sediment fractionation within the flow and/or muddy substrate acquisition. Their presence and character within hydrocarbon reservoirs are important because of their impact on recoverable volumes and reservoir performance. The Castagnola system of NW Italy represents the fill of a fully ponded basin containing both turbidites and mudclast-rich HEBs. Because mud cannot be lost down-dip, measured bed thicknesses (sandstone and mudcap) give a good approximation of the final sediment budget of the depositing flows. Moreover, two distinct (arkosic vs. lithic) sediment sources can be recognised in both turbidites and HEBs, allowing petrographic signatures to be investigated. Results show that in the Castagnola system: (i) turbidites have a sandstone to mudcap thickness ratio of around 1:2, while HEBs have a ratio of around 2:1 (regardless of petrographic signature) and (ii) sandstone beds (both turbidites and HEBs) commonly show an unmixed petrographic signature, while mudcaps more frequently have a mixed character. The mixed signature of the mudcaps suggests that the flows incorporated significant amounts of mud en-route. The different sand to mudcap ratio of turbidites and HEBs indicates that the flows that deposited the turbidites likely incorporated more mud than those that deposited HEBs of similar thickness (the amount of mud present in the HEBs mud-rich division is not enough to explain such differences). This conclusion challenges the current paradigm that flows with greater erosional capacity are more likely to develop hybrid character, and may force a rethink regarding the criteria needed to predict the likelihood of HEB development in deep-water hydrocarbon reservoirs.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017