--> --> Revealing the Complexity of Heterogeneous Volcaniclastic Facies of Mid Miocene Onnagawa Formation, Akita Basin, Japan

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Revealing the Complexity of Heterogeneous Volcaniclastic Facies of Mid Miocene Onnagawa Formation, Akita Basin, Japan


Volcanic rocks exhibit numerous variations of features reflecting the complexity associated with volcanic edifices. Present study composed of such heterogeneous lithocolumn which need to be evaluated in light of high-resolution textural analysis in order to come up with robust lithofacies model. A detailed analysis of high-resolution images was used to define the associated lithofacies and morphofacies of the drilled lithocolumn. This simple methodology is broadly categorized in three major steps. The first step includes the identification of the image electrofacies using the processed microresistivity borehole images and dips, supplemented by observations from openhole logs, geochemical logs, and mud log descriptions. In the second step, the recognized sub-facies are interpreted in terms of depositional processes and conditions. The third step groups the identified sub-facies into distinct facies associations, which are predictors of certain depositional settings. After the successful characterization of image-based electro-facies, an integrated approach was adopted to link the various facies with elemental spectroscopy data to identify the relationship with the genesis and emplacement of various rock types. In the study well, the facies can be grouped into five major sections: FA1, FA2, FA3, FA4, and FA5. The basal part, FA1 of the study interval, is characterized mainly by explosive facies, consisting of base surge and pyroclastic deposits, FA2 is also dominated by explosive facies; an increase in calcium feldspar, with reduced potassium feldspar and very low clay across the extrusive facies interval. FA3 is characterized by a massive unit rich in calcium feldspar, dolomite, and pyroxene . The upper section of the study, well represented by FA4, is silica-rich at the bottom and magnesium-calcium-rich . This interval is characterized by the mixing of reworked volcanogenic sedimentary facies with extrusive facies. The uppermost part of the entire interval is represented by FA5 which is silica- and clay-rich, with strong evidence of a potassium feldspar. These identified facies were analyzed in the holistic vertical sequence in association with all other conventional well logs, such as nuclear magnetic resonance (NMR), acoustic, and triple combo, to build an in-depth understanding of the distribution of geological facies in the major volcanic edifices.