--> Using UAV Photogrammetry to Quantify Differential Cementation of Turbidite Sandstones: An Example From the Gremiasco System (Upper Oligocene, Tertiary Piedmont Basin of NW Italy)

AAPG ACE 2018

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Using UAV Photogrammetry to Quantify Differential Cementation of Turbidite Sandstones: An Example From the Gremiasco System (Upper Oligocene, Tertiary Piedmont Basin of NW Italy)

Abstract

Diagenesis plays a key role in controlling distribution of porosity and permeability of sedimentary rocks. Understanding type, timing of formation and spatial distribution of cements is thus important for predicting performance of hydrocarbon reservoirs. In this study Unmanned Aerial Vehicle (UAV)-based photogrammetry is used to assess the differential cementation of a channel-fill turbidite sandstone body (Gremiasco System of NW Italy, Upper Oligocene). The studied outcrop is part of an up to 40m-thick string-like sandstone body of c. 200m across representing the infill of a low-sinuosity slope channel entrenched into hemipelagic marlstones. Diagenesis resulted in an uneven cementation with development of cm to dm-scale calcite-cemented concretions scattered in a host sandstone lacking calcite cement. Because concretions are less prone to weathering, they stand out from the outcrop providing the opportunity to investigate their shape, topology and volume fraction. The adopted workflow entails: i) UAV acquisition of high resolution photographs and extraction of a dense point cloud based on photogrammetry principles; iii) modelling of a meshed outcrop replica textured with photographs; iii) digitization of concretions vs. host sandstones from the outcrop model, and iv) core sampling for investigating texture, grain composition, diagenetic features and oxygen and carbon isotopic signature of calcite cements. Facies analysis indicates an early-stage fill comprised of amalgamated beds of mud clast-rich gravelly sandstones and a last-stage fill dominated by turbidites with partially preserved mud caps. Analysis of shapes extracted from digitization highlights spheroidal concretions with flat-lying major axes, aspect ratios in the range 1-5 and a cumulative volume of c. 20% of the whole channel-fill volume. Moreover, there is a hint of correlation between concretion shapes, facies and their spatial association to mud clasts and mud caps. The interparticle pore-space of concretions is tightly filled by shallow burial, blocky calcite precipitated after mechanical compaction, whereas host sandstones are cemented by authigenic smectite clay minerals and show up to c. 10% of preserved post-compaction porosity. Lastly, isotopic signature of calcite cements indicates sourcing of calcium carbonate from the hemipelagites forming the channel substrate, which provided most of the mudstone found within the channel-fill in form of clasts or capping turbidite beds.