Abstract
Gravity-flow can carry a large number of sediments and organic matters from shallow water to deep lakes with its strong transporting energy, directly or indirectly facilitating the formation of deep-water tight reservoirs and shale reservoirs. Therefore, studying the genetic types, dynamic mechanisms, and depositional models of gravity-flow deposits is essential in the exploration of unconventional petroleum in large lacustrine basins. This research studied the genetic types, dynamic mechanisms, and sedimentary models of the gravity-flow deposits of the Chang 6 oil group in the Heshui Area, Ordos Basin, China, aiming to reveal its petroleum geological significance. Core observation, microscopic thin section identification, particle size analysis, and determination of rare earth elements were carried out. As a result, three types of gravity-flow deposits are detected, namely, slide-slump, sandy debris flow, and turbidity current. A certain slope gradient in bed form is the necessary geomorphic condition for gravity flow formation, and determines its development level, distribution range, and flow transformation efficiency. Sufficient provenance lays the material foundation and determines its depositional composition and development type. Other factors include earthquakes, volcanoes, and floods, which serve as triggering forces. In addition, fragmentation, liquefaction, and fluid mixing are the main dynamic mechanisms driving flow transformation. Based on the flow type of gravity flow, particle size characteristics, gravity-flow transformation relations, development mechanism, and spatial distribution pattern, we distinguished two depositional gravity-flow models, i.e., slump turbidite body and sublacustrine fan. Re-portrait the spatial distribution of deep-water gravity flow in the study area. From the perspective of sedimentology, explain the genesis of sand bodies in the northeast and southwest. The sandy debris flow in the middle fan braided channel microfacies of the sublacustrine fan sways the development of thick massive sand bodies in the study area. Hybrid event beds formed by the fluid transformation in a slump turbidite are the potential dessert area for deep-water tight oil and gas.
Highlights
Sand body of deep-water gravity-flow refers to gravity-driven sands deposited in deep-water areas below the storm wave base plane, which is currently a key area for unconventional petroleum exploration (Haughton et al, 2009; Zou et al, 2009; Zou et al, 2012; Cao et al, 2017)
The determination results of rare earth elements of nine rock samples from wells Le 36, Ban 33, Zhuang 37, and Zhuang 42 reveal that the REE distribution features right gentle inclination of rich in light rare earth (LREE) and depletion in heavy rare earth (HREE), with obvious negative Eu anomaly (Figure 4A)
Based on the results obtained from the core observations of several coring wells in the study area, the fluid rheological characteristics, and the methods employed to classify deep-water gravity-flow inside and outside China, we divided the gravity-flow in the study area into three main types: slide-slump featuring mass transport, sandy debris flow with plastic rheological properties, and turbidity current with Newtonian rheological characteristics
Summary
Sand body of deep-water gravity-flow refers to gravity-driven sands deposited in deep-water areas below the storm wave base plane, which is currently a key area for unconventional petroleum exploration (Haughton et al, 2009; Zou et al, 2009; Zou et al, 2012; Cao et al, 2017). In recent years, based on the research of marine deep-water gravity flow, great progress has been made in the studies on deep-water gravity-flow in terrestrial lake basins (Yang et al, 2014; Sonnenberg, 2017; Kvale et al, 2017; Pan et al, 2017; Perry et al, 2017; Wang et al, 2018). Better understandings of the genetic types, dynamic mechanisms, and depositional models of gravity-flow are conducive to deep-water oil and gas exploration
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