Genetic Feature Analysis And Log Evaluation Methods For Low Resistivity Reservoirs In Northern Dagang Area

According to the statistics a great deal of information at home and abroad, there are low-resistivity reservoirs(that is, low-resistivity reservoir, mainly referring to the reservoir and water resistivity similar to the rate) in most of the world's oil field, because the causes of low-resistivity reservoirs are complex. And logging tools and present log interpretation methods are imperfections making the low-resistivity reservoirs exploration and development become a worldwide problem. Northern Dagang-forming system between Huanghua depression and Qikou Depression, is a kit containing many of the major oil-field, due to the deposition, structure, fault, and other factors, makes difference of resistivity between oil-bearing reservoirs and water-bearing reservoirs is very small, also makes low resistivity to the formation of identification even more difficult. These are always puzzle workers of oil field.Many basic information of Northern Dagang area, such as lithology character, physical characteristics, water characteristics and clay minerals ,were analysisd in this paper , the causes of low-resistivity reservoirs in Northern Dagang area was defined. There are five main types, such as highwater salinity, clay additional conductivity, high bound water saturation, micro-cracks and rock skeleton of conductive. In the important region ,Gang-dong and wing of Gang-bei there are two main types of low-resistivity reservoirs, additional conductive type of clay and high bound water saturation type. The water salinity is low in Guantao formation, so the cause of low-resistivity mainly is additional conductivity of the clay, but the lack of cation exchange of data, it is not sufficient to prove their clay conducting additional role of size. With the deepening of depth, water salinity in Dongying Group and Shahejie Group is bigger than Guantao Group, cause of low-resistivity was mainly due to highbound water saturation, and the high bound water saturation divided into two categories, the first is high clay irreducible water saturation, and the other is the high silt content micro-fine pore irreducible water saturation. Most of these low-resistivity reservoirs is due to the formation of fine-grained components of micro-pores, but there are also high clay bound water saturation of the type.The sedimentary faces of Northern Dagang area was analysed in this paper, also the relationship between sedimentary faces and distribution of oiland gas was summed up .The principal sedimentary faces of Dongying group in study area are delta plain and delta front of the mouth and the dam, the development of sand, covered storage conditions are better, and tectonic rational allocation and feasible as a good reservoir. The principal sedimentary faces of Shahejie group in study area is alluvial fan, mainly because of fewer land-based sources of debris, water energy is weak, smaller lithology, it is easier to form low resistance reservoirs. Lithology changes in the relative deposition played an important role in low-resistivity often in the reservoir lithology smaller fine sandstone, shale and siltstone in the formation of the heavier, and special sedimentary environment - delta front sand in the area of low resistivity reservoir laid the foundation for the formation.For different cause of low-resistivity, lots of logging information of Gang-dong and wing of Gang-bei area, such as properties, water characteristics, press mercury, grain size, test oil and so on, were used to built up the plate of porosity and resistivity , porosity and resistivity millisecond method and other logging, gas measurement imformation were integrated to identify the low-resistivity oil reservoirs.Three-pore conductivity model was used for lows resistance reservoirs interpretation, this model divides reservoir pore into free fluid pore,micro-capillary pore and clay-water pore. Dagang Oilfield Exploration and Development Institute provide the core analysis of various data and logging data, the data were used to calculate the total porosity, clay content and several parameters of the model, and porosity and clay content were matching the core analysis data, but due to the lack of clay analysis data, so the contribution of clay-water pore is indeterminacy. According to various regions and different laboratories`s rock-resistivity parameters, different cementation index were used, according to different regions, the different parameters were adjusted to make the results more in line with the actual formation of the results.In short, we get a set of systemic log interpretation method from the course of calculated reservoir parameters after defining the causes of low resistance reservoirs in Northern Dagang area in Dagang oil field. By porosity-resistivity method and three-pore conductivity model method, low-resistivity reservoirs problems were dealt in Northern Dagang area of Dagang oil field, then better interpretation effect is got.