Overall PSD and Fractal Characteristics of Tight Oil Reservoirs: A Case Study of Lucaogou Formation in Junggar Basin, China

Lucaogou tight oil reservoir, located in the Junggar Basin, Northwest of China, is one of the typical tight oil reservoirs. Complex lithology leads to a wide pore size distribution (PSD), ranging from several nanometers to hundreds of micrometers. To better understand PSD and fractal features of Lucaogou tight oil reservoir, the experiment methods including scanning electron microscope (SEM), rate-controlled mercury injection (RMI) and pressure-controlled mercury injection (PMI) were performed on the six samples with different lithology. The results indicate that four types of pores exist in Lucaogou tight oil reservoir, including dissolution pores, clay dominated pores, microfractures and inter-granular pores. A combination of PMI and RMI was proposed to calculate the overall PSD of tight oil reservoirs, the overall pore radius of Lucaogou tight oil reservoir ranges from 3.6 nm to 500µm. The fractal analysis was carried out based on the PMI data. Fractal dimension (Fd) values varied between 2.843 and 2.913 with a mean value of 2.88. Fd increases with a decrease of quartz content and an increase of clay mineral content. Samples from tight oil reservoirs with smaller average pore radius have stronger complexity of pore structure. Fractal dimension shows negative correlations with porosity and permeability. In addition, fractal characteristics of different tight reservoirs were compared and analyzed

Occurrence characteristics and influencing factors of movable fluid in the medium- and low-permeability reservoirs of the Es32+3 submember of the Gao3102 fault block in the Gaoshangpu Oilfield

Objective In order to clarify the occurrence characteristics of movable fluids in medium- and low-permeability reservoirs, so as to better guide the increase in oil reserves and production in the oilfield. Methods This study focuses on the medium- and low-permeability sandstone reservoirs in the Es32+3 submember of the Gao3102 fault block in the Gaoshangpu Oilfield. Based on thin section data, whole-rock diffraction analysis, high-pressure mercury injection curves, closed core nuclear magnetic resonance data, scanning electron microscopy, and oil-water phase permeability curves, the occurrence characteristics and influencing factors of movable fluids in reservoirs with different pore structures were conducted. Results The results show that (1) the movable fluid saturation of reservoirs with different pore structures varies greatly. The T2 spectrum of the nuclear magnetic resonance of the class Ⅰ medium porosity and medium throat reservoir is bimodal with left low and right thigh, and the average saturation of movable fluid is 61.14%. The T2 spectrum of the nuclear magnetic resonance of the class Ⅱ medium and small pore fine throat reservoir shows a double peak type with left high and right high, and the average of movable fluid saturation is 45.24%. The T2 spectrum of the nuclear magnetic resonance of the class Ⅲ fine porous micro throat reservoir shows a double peak type of left high and right low, and the average of movable fluid saturation is 30.45%. The T2 spectrum of the nuclear magnetic resonance of the class Ⅳ microporous micro throat reservoir shows a left high single peak, and the average saturation of movable fluid is 13.86%. (2) The macroscopic physical properties, microscopic pore structure, clay mineral content and reservoir wettability of the reservoir jointly control the occurrence characteristics of the movable fluid. Among them, the microscopic pore structure is a key factor in the occurrence of movable fluid. The larger the pore throat radius is, the better the reservoir seepage capacity, the lower the clay mineral content is, the weaker the reservoir wettability is, and the higher the movable fluid saturation is. Conclusion The results of this research can provide a reasonable scientific basis for high-efficiency water injection development of the Es32+3 reservoir in the Gaoshangpu Oilfield

Progress and prospects of reservoir development geology

This paper deals with the main scientific problems, academic connotation, progress and prospects of reservoir development geology. The reservoir development geology involves the key scientific problems of reservoir connectivity, flow ability, and time variability. Its research focuses on the forming mechanism and distribution model of geological factors controlling the reservoir development, the control mechanism of geological factors to oil and gas production, the rule of reservoir dynamic evolution during development, and the reservoir characterization and modeling technology. Important progress has been made on theory and technology of reservoir development geology in high water-cut reservoirs, low permeability and tight shale reservoirs, fracture-cavity reservoirs, which makes the reservoir development geology grow as an independent academic subject already. With the development expansion in areas of deep-strata, deep-water, and unconventional hydrocarbon reservoirs, and the increasing difficulties of high water-cut reservoir development, the theory and technology of reservoir development geology remain to be developed in order to support efficient and economic development of hydrocarbon fields with a sustainable growth. Key words: development geology, scientific problem, research progress, development geologic factor, reservoir developmen

Investigating Influential Factors of the Gas Absorption Capacity in Shale Reservoirs Using Integrated Petrophysical, Mineralogical and Geochemical Experiments: A Case Study

Estimating in situ gas content is very important for the effective exploration of shale gas reservoirs. However, it is difficult to choose the sensitive geological and geophysical parameters during the modeling process, since the controlling factors for the abundance of gas volumes are often unknown and hard to determine. Integrated interdisciplinary experiments (involving petrophysical, mineralogical, geochemical and petrological aspects) were conducted to search for the influential factors of the adsorbed gas volume in marine gas shale reservoirs. The results showed that in shale reservoirs with high maturity and high organic content that the adsorbed gas volume increases, with an increase in the contents of organic matter and quartz, but with a decrease in clay volume. The relationship between the adsorbed gas content and the total porosity is unclear, but a strong relationship between the proportions of different pores is observed. In general, the larger the percentage of micropores, the higher the adsorbed gas content. The result is illuminating, since it may help us to choose suitable parameters for the estimation of shale gas content

Impacts of the Base-Level Cycle on Pore Structure of Mouth Bar Sand Bars: A Case Study of the Paleogene Kongdian Formation, Bohai Bay Basin, China

The pore structure of rocks can affect fluid migration and the remaining hydrocarbon distribution. To understand the impacts of the base-level cycle on the pore structure of mouth bar sand bodies in a continental rift lacustrine basin, the pore structure of the mouth bar sand bodies in the ZVC (ZV4 + ZV5) of the Guan195 area was studied using pressure-controlled mercury injection (PMI), casting sheet image and scanning electron microscopy (SEM). The results show that three types of pores exist in ZVC, including intergranular pores, dissolution pores, and micro fractures. The porosity is generally between 1.57% and 44.6%, with a mean value of 19.05%. The permeability is between 0.06 μm2 and 3611 μm2, with a mean value of 137.56 μm2. The pore structure heterogeneity of a single mouth bar sand body in the early stage of the falling period of short-term base-level is stronger than that in the late stage. During the falling process of the middle-term base level, the pore structure heterogeneity of a late single mouth bar sand body is weaker than that of an early single mouth bar sand body. In the long-term base-level cycle, the pore structure heterogeneity of mouth bar sand bodies becomes weaker with the falling of the base-level

Analysis of Volcanic Development Model and Main Controlling Factors of Oil Distribution in the Third Member of Shahejie Formation in Zaoyuan Oilfield

In order to clarify the influence in the volcanic mode and structure on the oil reservoirs, the volcanic reservoir characteristics, volcanic eruption pattern, and volcanic eruption period of the third member of the Shahejie formation in the Dagang Oilfield Zao35 fault block are studied by combining logging, 3D seismic, and production data, and to provide geological basis for the subsequent development of volcanic reservoirs. The results show that the volcanic body of the Zao35 fault block is jointly controlled by the fissure-centered eruption mode, and there are three strings of bead-shaped eruption centers as well as a fault overflow channel. Based on the seismic response characteristics, the volcanic rocks can be divided into three main eruption cycles. Moreover, combined with the relatively stable mudstone interlayer encountered by the single well, it can be further divided into eight volcanic eruption periods. There are three different lava units in the overflow facies of each stage, namely thick layer dense basalt, basalt with pores developed at the top and dense at the bottom, and basalt with pores developed at the top and dense in the middle. Influenced by the volcanic development model, the oil is mainly distributed in the volcanic reservoir on the slope belt between the central eruption and the fracture eruption. Affected by volcanic eruption periods, the pore basalt at the top and bottom of each period controls the distribution of the oil reserves. Therefore, the thick layer stomatal basalt located between the two eruption modes is the key target of the next development

Controlling mechanism of pore-throat structure of different lithofacies on gas-water relative permeability characteristics of tight sandstone gas reservoir: A case study of the Lower Shihezi Formation in the Well J72 block of the Dongsheng Gas Field, Ordos Basin

To understand the controlling mechanism of pore-throat characteristics on gas-water relative permeability characteristics of tight sandstone gas reservoirs and better guide the increase in gas reserves and production, taking the Lower Shihezi Formation reservoir in the J72 well block of the Dongsheng Gas Field as a research object, the effects of microscopic pore-throat parameters on gas and water transport properties were studied by combining various analytical data and lithofacies division. The controlling effect of rock lithofacies on gas content and productivity was revealed.Results show that the lithofacies of the Lower Shihezi Formation in the J72 well block can be classified into conglomerate rock, gravelly sandstone, pebbly coarse-grained sandstone, medium- and fine-grained sandstone, and mudstone lithofacies, of which the pebbly coarse-grained sandstone and gravelly sandstone lithofacies are the high-quality lithofacies. Pore types are mainly secondary intergranular dissolved pores, with an average proportion of 48.5%. The proportion of intergranular dissolved pores in pebbly coarse-grained sandstone lithofacies, gravelly sandstone, and medium- and fine-grained sandstone lithofacies decreases successively.Pore space in the three lithofacies is multifractal, and the proportion of large pores gradually decreases. The high-quality lithofacies has superior physical properties and small comprehensive fractal dimensions. The high-quality lithofacies also has low clay mineral content, large average pore-throat radius, large movable gas porosity, maximum effective gas permeability during gas transport, and high gas-bearing capacity. A comprehensive study shows that high-quality lithofacies is the key factor in controlling the high and stable production of gas wells. The in-depth analysis of the microscopic characteristics of lithofacies provides useful guidance for the efficient development of tight sandstone gas reservoirs

Paleokarst Reservoir Features and Their Influence on Production in the Tahe Oilfield, Tarim Basin, China

This study analyzed the characteristics of cave heights and fillings, influences on cave distribution by unconformity and geomorphology, and production control trend for geological factors. The heights and numbers of caves in different geomorphologies are basically similar. The cave heights are greater near the unconformity surface. On an average, there are higher degrees of cave development and larger cave heights in the karst slopes than in the karst highlands and karst depression. Based on core observations, four types of reservoirs were identified: unfilled caves, filled caves, open fractures and filled fractures. The formation of caves is mainly controlled by faults and underground water surfaces. Faults works obviously in the karst highland, while underground water surfaces obviously in the karst slope. Caves controlled by faults are mostly unfilled, while controlled by underground water surfaces are mostly filled with clastic sediments. According to the results from both first-day and cumulative production, production is not highly correlated with cave heights, but rather storage and connectivity of caves and fractures. Generally, wells with unfilled caves or open fractures have higher first-day and cumulative production than filled caves or fractures. The distribution of high first-day and cumulative production is consistent overall