Tectonically controlled lakes developed during Miocene lateral extrusion of the Eastern Alps. Mineralogy, and the inorganic and organic geochemistry of rocks from three boreholes were investigated to reconstruct the evolution of Lake Ingering and Lake Groisenbach and to study the distribution of source rocks in pull-apart basins. Gas-prone coal and oil-prone sapropelic shale accumulated during the initial, shallow stages of Lake Ingering. Thereafter, the lake deepened rapidly. 125-m-thick prodelta shale containing a type II kerogen was deposited in the brackish, several hundred meter deep, hydrologically closed lake. Afterwards, decreasing subsidence allowed the filling of the lake by prograding deltas. During the advance of the deltaic systems, the lake became shallower, hydrologically open, and the brackish influence terminated. Source rock quality decreased significantly during the filling stage of the lake, a consequence of dilution of autochthonous organic matter and of enhanced input of land plants. Despite its considerable dimensions, formation and filling of Lake Ingering lasted only two million years. Lake Groisenbach was considerably smaller and more susceptible to high-frequency changes in lake chemistry. Although the water body was temporarily oligosaline, brackish conditions did not occur. High sulphur contents were due to anoxic events and the inflow of Ca-rich waters. Abundant dissolved silica favoured diatoms blooms. 相似文献
A suite of reservoir cores (oil sands) from a single well in Bohai Bay Basin, East China, displayed a progressive increase in petroleum biodegradation extent on the basis of bulk composition and 25-norhopane content. This fits with the proposal that subsurface petroleum biodegradation is dominantly an anaerobic process and usually occurs at the oil–water contact. It is likely that sequential microbial degradation of hydrocarbons under anoxic conditions does not occur in a true stepwise fashion, but is controlled by various factors such as concentration and solubility of hydrocarbons and their diffusion rate to the oil/water contact. In fact, 25-norhopanes were formed prior to the complete elimination of the acyclic, and mono- and bicyclic alkanes. An inverse response of the 22S/(22S + 22R) ratio between each extended 17α(H)-hopane and its corresponding 25-norhopane was observed as severe biodegradation occurred, supporting the proposal that the 25-norhopanes originate from demethylation of hopanes. Field observation revealed that biomarkers without extended alkyl side chains, such as oleanane, gammacerane and β-carotane, have significant resistance to biodegradation and can be used as naturally occurring “internal standards” to evaluate variations in other biomarkers. The results suggest that the quantity of 25-norhopanes showed a minor increase as the hopanes decreased significantly, i.e. only partial hopane conversion to the corresponding 25-norhopanes. Alternative degradation pathways for hopanes might occur in reservoirs, in addition to C-25 demethylation. 相似文献
The Sebei gasfield is the largest biogas accumulation found in China and many reservoirs and seal rocks superposed on a syndepositional anticline in Quaternary. The biogas charging and dissipating process and its distribution have been a research focus for many years. The authors suggest a diffusing and accumulating model for the biogas, as they find that the shallower the gas producer, the more methane in the biogas, and the lighter stable carbon isotope composition of methane. Based on the diffusing model, diffused biogas is quantitatively estimated for each potential sandy reservoir in the gasfield, and the gas charging quantity for the sandy reservoir is also calculated by the diffused gas quantity plus gas reserve in-place. A ratio of diffusing quantity to charging quantity is postulated to describe biogas accumulating state in a sandy reservoir, if the ratio is less than 0.6, the reservoir forms a good gas-pool and high-production layer in the gasfield, which often occurs in the reservoirs deeper than 900 m; if the ratio is greater than 0.6, a few gas accumulated in the reservoir, which frequently exists in the reservoirs shallower than 900 m. Therefore, a biogas accumulation model is built up as lateral direct charging from gas source for the sands deeper than 900 m and indirect charging from lower gas-bearing sands by diffusion at depth shallower than 900 m. With this charging and diffusion quantitative model, the authors conducted re-evaluation on each wildcat in the central area of the Qaidam Basin, and found many commercial biogas layers.
