Benzhong Xian

Delta-fed turbidites in a lacustrine rift basin: the Eocene Dongying depression, Bohai Bay Basin, East China

ABSTRACT Sediment avalanche from delta ramp is one of the significant development mechanisms for a turbidite system in a lacustrine basin. To advance our understanding of deep-water sedimentary processes in a lacustrine delta ramp, delta-fed turbidites in the Eocene Dongying depression of the Bohai Bay Basin were studied using core data, 3-D seismic data and well log data. Sandy debris flows, muddy debris flows, mud flows, turbidity currents, slides, sandy slumps and muddy slumps were interpreted based on the identification of lithofacies. Data indicates that deep-water sedimentary processes in the study area were dominated by debris flows and slumps, which accounted for ∼68% and 25% (in thickness) of total gravity flow deposits, respectively; turbidity-current deposits only accounted for ∼5%. Mapping of turbidites showed that most were deposited after short-distance transportation (<20 km), restricted by the scale of deep-water areas and local topography. Channels, depositional lobes, debris flow tongues, muddy turbiditic sheets, slides and slumps were identified in a delta-fed ramp system. Slides and slumps were dominant at the base of slopes or at the hanging walls of growth faults with strong tectonic activity. Channels and depositional lobes developed in gentle, low-lying areas, where sediments were transported longer distances. Sand-rich sediment supply, short-distance transportation and local topography were crucial factors that controlled sedimentation of this ramp system. Channels generally lacked levees and only produced scattered sandstones because of possible hydroplaning of debris flow and unstable waterways. In addition to lobes, debris flow tongues could also be developed in front of channels. These findings have significant implications for hydrocarbon exploration of deep-water sandstone fed by deltas in a lacustrine basin.

Identification of hydrovolcanism and its significance for hydrocarbon reservoir assessment: A review

Investigations of modern volcanic eruptions have demonstrated that ancient volcanic eruptions widely involved water, which was thus named as hydrovolcanic eruptions. Hydrovolcanisms are distinctive in many aspects, such as dynamics, eruptive pattern, texture and structure of rock, and vesicularity. First, normal sediments covered directly by volcanic rocks are the indicators of eruption environments. In addition, microfeatures, special structures, lithofacies or facies associations, and geochemical index of volcanic rocks can also provide significant evidences. Moreover, perlitic texture, quenching fragmentation, surface feature, cementation type, vesicularity, and pillow structure, parallel bedding, large-scale low-angle cross-bedding, antidune cross-bedding of pyroclast are keys to indicating hydrovolcanisms. Clearly, these marks are not equally reliable for identifying eruption environments, and most of them are effective and convincible in limited applications only. For explosive eruptions, the most dependable identification marks include quenching textures, vesicularity in pyroclasts and special large-scale cross-bedding. However, for effusive eruptions, useful indicators mainly include pillow structure, peperite and facies associations. Condensation rate of magma, exsolution of volatile affected by eruptive settings and magma−water interaction, and quenching in hydrovolcanisms have an influence on formation and scale of primary pores, fractures and their evolution during diagenetic stage. Therefore, this review provides systematic identification marks for ancient hydrovolcanisms, and promotes understanding of the influence of eruptive environments on hydrocarbon reservoirs of volcanic rocks in oil−gas bearing sedimentary basins.

Mapping sediment-dispersal characteristics of Neogene deltas using seismic geomorphology, Liaodongdong area, Bohai Bay Basin, China

ABSTRACT Seismic geomorphology has provided effective technical support for the study on the sediment dispersal system in poorly drilled area based on seismic data. In this paper, taking the Neogene strata of eastern Liaodong region in the Bohai Bay Basin, eastern China, as an example, strata slicing was performed in the 3D seismic data volume with phase adjustment under the constraints of a few drilling and logging data, which aims to depict the planar distribution and evolution of the sandbodies of Guantao–Minghuazhen Formation layer by layer. One hundred and seventy-five isochronous strata slices of amplitude attribute were extracted in the time domain, which had greatly improved the quality of sandbody planar distribution map and provided an effective method for analyzing the sedimentary microfacies of different periods and their evolution in the study area.