Facies architecture of Paleogene subaqueous trachytes and its implications for hydrocarbon reservoirs in Liaohe Basin

Key words:  trachyte  volcanic facies  facies architecture  subaqueous eruption  volcanic reservoir  Liaohe Basin

Abstract:

The Paleogene trachytes in the Eastern Sag of Liaohe Basin were dominantly erupted and emplaced in subaqueous settings. Borehole data and seismic profiles were used to study the facies classification, sequence and architecture of these trachytic rocks, and to reveal the distribution of high-quality reservoirs and their controlling factors. Five principal volcanic facies and eleven sub-facies can be identified in the trachytic successions by means of facies analysis on both boreholes and cross-sections. A representative facies model was proposed for volcanic facies analysis and prediction, which reveals three volcanic facies associations corresponded to the proximity to volcanic conduit. Respectively, the central-proximal facies association is dominated by extrusive lava rocks, while the medial facies association is characterized by pyroclastic flow deposits, and the distal facies association is composed of water-settled fall deposits and volcaniclastic deposits. Two vertical facies sequences from bottom to top are explosive to extrusive facies and volcanic conduit to extrusive facies, respectively. Lateral facies sequence from volcanic center to edge can be described as the volcanic conduit and extrusive facies combination, the effusive and explosive facies combination, and the volcaniclastic facies. Quenching fragmentation and volcanic explosion resulted from mingling of magma and water are considered to be the key influencing factors to the characteristics of volcanic facies and reservoirs of subaqueous trachytes in the study area. Intra-particle dissolution pores, residual inter-particle pores and the related inter-particle cement dissolution pores are proven to be the most important valid porosities in trachytic reservoirs. The intra-particle phenocrystal and matrix dissolution pores, especially, make the major contribution to reserve capacities which account for 53.2％ in total, while the permeability is determined by tectonic fractures. Three principal types of high-quality reservoirs with average porosity larger than 10.3％ and permeability larger than 0.35×10-3 μm² are developed in outer extrusive sub-facies, pyroclastic flow sub-facies and volcanic diatreme sub-facies, which should be taken as the major exploration targets of the subaqueous trachytic successions.