Current oil saturation in the Moxizhuang (莫西庄) Oil Field in central Junggar (准噶尔) basin was evaluated by logging interpretation and measured on core samples, and the paleo-oil saturation in both the pay zones and water zones was investigated by grain-containing-oil inclusion (GOI) analysis. The pay zones in this field have low oil saturation and display low resistivity and small contrast between pay zones and water zones, and are classified as low-porosity, low oil saturation, and low resistivity reservoirs. Both the current low oil-saturation pay zones and the water zones above 4 365 m have high GOI values (up to 38%), suggesting high paleo-oil saturation. The significant difference between current oil saturation from both logging interpretation and core sample measurement and paleo-oil saturation indicated by GOI analysis suggests that this low oil-saturation field evolved from a high oil-saturation pool. Lateral re-migration and spill of formally trapped oil owing to changes in structural configuration since Neogene was the most plausible mechanism for oil loss in the Moxizhuang Oil Field. The combined effects of differential accumulation in the charge phase and the differential re-migration and spill of accumulated oil in Neogene are responsible for the complicated correlation between residual oil saturation and porosity/permeability of the reservoir sandstones and the distribution of low oil-saturation pay zones and paleo-oil zones (current water zones).
The Damintun (大民屯) depression,a small (about 800 km2 in area) subunit in the Bohai (渤海) Bay basin,hosts nearly 2×108 t of high-wax oils with wax contents up to 60%. The high-wax oils have high consolidation temperatures and viscosities. The high-wax oils were generated from the fourth member of the Shahejie Formation (Es4),which is also important source rocks for oils in other subunits of the Bohai Bay basin. Yet high-wax oils have not been found in significant volumes elsewhere in the Bohai Bay basin. Geological conditions favourable for high-wax oil enrichment were studied. This study shows that the unusual concentrations of high-wax oils in the depression seem to result from at least three different factors:(1) the presence of organic-matter rich source rocks which were prone to generate wax-rich hydrocarbons; (2) the formation of early overpressures which increased the expul-sion efficiency of waxy hydrocarbons; and (3) reductions in subsidence rate and basal heat flows,which minimized the thermal cracking of high molecular-weight (waxy) hydrocarbons,and therefore pre-vented the high-wax oils from being transformed into less waxy equivalents.
Secondary petroleum migration in the eastern Pearl River Mouth basin was modeled using the three-dimensional PATHWAYSTM model,which assumes that the positions of petroleum mi-gration pathways are controlled by the morphology of the sealing surfaces. The modeling results have accurately predicted the petroleum occurrences. Most commercial petroleum accumulations are along the predicted preferential petroleum migration pathways (PPMP),and most large fields (petroleum re-serves greater than 1×108 t) have more than one preferential petroleum migration pathways to convey petroleum to the traps. The lateral migration distance for oil in the LH11-1 field,the largest oilfield so far discovered in the Pearl River Mouth basin,was more than 80 km. The case study suggests that in lacustrine fault basins,petroleum can migrate over a long distance to form large oilfields without driving force from groundwater flow. The focusing of petroleum originating from a large area of the generative kitchens into restricted channels seems to be essential not only for long-range petroleum migra-tion in hydrostatic conditions,but also for the formation of large oil or gas fields. The strong porosity and permeability heterogeneities of the carrier beds and the relatively high prediction accuracy by a model that does not take into consideration of the effect of heterogeneity suggest that the positions of petroleum migration pathways in heterogeneous carrier beds with relatively large dipping angles are determined primarily by the morphology of the sealing surfaces at regional scales.
