Gravity flows may be triggered by different initiation processes in both marine and lacustrine basins. Recognizing the different initiation processes of gravity flow based on their deposits is vital to accurately establish gravity-flow sandstone distribution, which is important for defining paleogeography and for efficient oil and gas exploration. Gravity-flow deposits in the Dongying sag were analyzed using three-dimensional seismic, well-log, grain size, and porosity and permeability data, along with core descriptions. Eleven lithofacies, nine bed types, and six bed-type associations were recognized in the gravity-flow deposits in the Dongying sag. Gravity-flow deposits around well Niu-110 were caused by delta-fed sediment failure. These deposits are characterized by medium to very fine-grained sandstone, abundant liquefaction and soft-sediment deformation structures, and thick laminae rich in plant debris. They formed massive sandstones accompanied by normally graded sandstone and lenticular-shaped sandbodies and are composed of chaotic deposits and tongue lobes. The above features collectively are indicative of typical collapsed-sediment transport to deep water by slumping and poorly cohesive debris flow to low-density turbidity current. Gravity-flow deposits around well Shi-100 are interpreted to have been caused by flooding river-fed hyperpycnal flows. These deposits are characterized by gravel to very fine-grained sand, abundant erosional structures and climbing ripples, and thin laminae rich in plant debris. They formed massive sandstone with some space stratification accompanied by inverse-then-normal grading sandstone and elongate or fan-shaped sandbodies and are composed of channel-levee systems and lobes. Stratified hyperpycnal flow is prone to form a hydraulic jump at the slope break. After the hydraulic jump, coarse-grained sediments were transported to the basin under the drag and shear of the upper part of the suspension flow. Gravity-flow deposits caused by flooding river-fed hyperpycnal flow are better reservoirs than those caused by delta-fed sediment failure under the same conditions. This study offers insight into the recognition criteria and flow processes of gravity flows caused by the different initiation processes in a lacustrine basin.
Siderite is a mineral resource and a climate sensitive mineral. Siderite nodule layers are widely developed in coal measures within the marine-ontinental transitional facies of North China epicontinental sea basin, yet their formation environment and genetic mechanism are poorly understood. In this paper, taking the siderite nodule layers in the coal measures of the Late Paleozoic Taiyuan Formation in Zibo area of North China as an example, the formation environment and genetic mechanism of siderite nodules in the transitional facies of the epicontinental sea basin are studied through detailed petrological, sedimentological and geochemical analysis. It is found that the siderite nodules in the research area were formed in a tidal flat-lagoon environment. The siderite nodules are formed in the synsedimentary stage, the original information of chemical composition and characteristics of the nodules being largely retained. Most of the carbon in the siderite nodules originates from inorganic carbon from marine carbonate rocks, with the remainder originating from inorganic carbon formed by dissimilatory iron reducing bacteria (DIR) degrading organic matter. Meanwhile, most of the iron in the siderite nodules originates from hydrothermal fluids, and the remainder from terrigenous sediments. The content of iron input from hydrothermal fluids and terrestrial sediment represents a trade-off relationship. The two genetic mechanisms of the siderite nodules are as follows: 1) Chemical genetic mechanism: The siderite nodules are mainly formed by the combination of low-valence iron (Fe2+) and inorganic carbon in seawater through reduction with iron input from hydrothermal fluids as the main source. 2) Biochemical genetic mechanism: Some is formed by the combination of low-valence iron (Fe2+) and inorganic carbon with high-valence iron (Fe3+) as an oxidant to degrade the organic matter and convert organic carbon into inorganic carbon through DIR. On this basis, a genetic model of the Late Paleozoic coal measures siderite nodules in the epicontinental sea basin is established.
A series of methane sorption isotherms were measured at 303 K, 313 K, 323 K, 333 K, and 343 K at pressures up to 12.0 MPa for two shale samples from the Upper Triassic Chang 7 Member in the southeastern Ordos Basin with total organic carbon content values of 5.15% and 4.76%, respectively. Both the Langmuir- and Dubinin–Radushkevich-based excess sorption models were found to well represent the excess sorption isotherms within the experimental pressure range. The maxima of absolute methane sorption capacity fitted by both models are not significantly different. In the current study, the effects of temperature and pressure on methane sorption capacity support the findings that under isothermal condition, methane sorption capacity of organic shale goes up with increasing pressure and under isobaric condition, while it goes down with increasing temperature. Good negative linear relationships between temperature and maximum sorption capacity exist both in the Langmuir and the Dubinin–Radushkevich models. In addition, a good positive linear relation exists between the reciprocal of temperature and the natural logarithm of Langmuir pressure, which indicate that temperature and pressure are really important for methane sorption capacity. The extended Langmuir and Dubinin–Radushkevich models have been improved to calculate the methane sorption capacity of shales, which can be described as a function of temperature and pressure. By means of using the two estimation algorithms established in this study, we may draw the conclusion methane sorption capacity can be obtained as a function of depth under geological reservoir. Due to the dominant effect of pressure, methane sorption capacity increases with depth initially, till it reaches a maximum value, and then decrease as a result of the influence of increasing temperature at a greater depth. Approximately, the maximum sorption capacity ranges from 400 m to 800 m.
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