Luofu Liu

Oil and gas accumulations in the Ordovician carbonates in the Tazhong Uplift of Tarim Basin, west China

Oil and gas fields of the Paleozoic cratonic basin in Tarim are mainly distributed in the Tabei and the Tazhong Uplifts. In the Tazhong Uplift, there are three sets of oil and gas stratum systems, of which the Ordovician traps are the largest in size and close to oil source beds with the most favorable conditions to form large-scale oil and gas fields. The Ordovician sediments in the Tazhong Uplift have experienced a stable platform and tableland margin development period. The Ordovician is divided into mudstone interval, interbedded mudstone and limestone interval, limestone interval, limestone and dolomite transitional interval and dolomite interval. Of these intervals, both interbedded mudstone and limestone interval and limestone interval contain source rocks, while limestone interval and dolomite interval are reservoir beds. Fault activities and denudation have an obvious impact on carbonate rock reservoirs. The major Ordovician trap types of the Tazhong Uplift include compressional fault block, fault anticline and buried hill, and regionally. The Tazhong Uplift can be generally divided into central fault horst trap belt, northern slope trap belt and southern slope trap belt. In view of the in-situ oil source in the Tazhong, oil and gas migration in the Tazhong Uplift is characterized by a short distance migration through sand bodies and unconformity surfaces laterally and along faults vertically. Based on this study, it is concluded that the favorable oil and gas accumulation zone should be the northern slope and the northwestern pitchout end of the uplift because these areas are located in the hydrocarbon migration pathway with weak tectonic activities and favorable preservation conditions.

Differential Hydrocarbon Migration and Entrapment in the Karstified Carbonate Reservoir: A Case Study of the Well TZ83 Block of the Central Tarim Uplift Zone

Abstract The hydrocarbon migration and entrapment mechanism in the lower overlapped basins, occurring in the complex carbonate pore-fissure-fracture reservoirs, is one of the key problems that have to be solved for effective hydrocarbon exploration. The production, gas/oil ratio, and the composition of crude oils and natural gas in the TZ83 Well block are high at the intersection point of the NE and NW-strike faults and decrease gradually along the ridge of the structure. A basic model of the pore-fissure-fracture system is built according to the achievements in the research on carbonate karstification. Processes of hydrocarbon migration and entrapment in this system are analyzed, which indicate that an understanding of the complexness of differential hydrocarbon migration is the key to interpreting this phenomenon. The hydrocarbon must charge the nearest compartment before migrating further away to charge other compartments in its pathway in the complex pore-fissure-fracture system. As a result, the following two phenomena appear: (1) Gas is enriched near the hydrocarbon injection point and drives away the oil, which is enriched in the compartments farther from the injection point. (2) The complex gas–oil–water relationship is controlled by the lateral connecting networks. Based on this, this article shows that the fault intersection point is the injection point of the oil and gas, and the main pathway system is distributed along the ridge of the structure. The theory of deferential hydrocarbon migration in the pore-fissure-fracture system can be presented from two aspects: (1) In hydrocarbon exploration, the structure of the fissure-fracture system should be described first, and then the special distribution of gas–oil–water can be predicted according to the main charging point and the main pathway system. (2) Exploration should be confined to the hydrocarbon charging point and the main pathway systems. An explored area should not be abandoned merely due to failures in some wells.

Geochemical characteristics of oils from Chepaizi Area, northwestern Junggar Basin, China

In recent years, great progress in petroleum exploration has been made obtained for Chepaizi Swell along the northwestern margin of Junggar Basin. But, the hydrocarbon sources of this area are still unknown, and this results in difficulty of determining the migration path system and migration trend. This also leads to difficulty in studying the oil-gas accumulation pattern of the area. In order to understand the oil sources, samples were systematically collected in the Swell and were analyzed for investigation of physical properties, chemical compositions and characteristics of bio-markers, carbon isotope and nitrogen compounds of the oils from Chepaizi. Through comparisons of physical and chemical characters, oils from different intervals of Chepaizi can be classified into two groups. One is the oils trapped in layers of Jurassic and below Jurassic (named as Lower Series of Strata oils), and another is those in layers of Cretaceous and above Cretaceous (named as Upper Series of Strata oils). Oils from the same group have similar densities, viscosities, group compositions and characteristics of saturated hydrocarbon, bio-marker and carbon isotope, while the oils from different groups are obviously different for the above-mentioned parameters. By fine oil-oil correlation, one can know that the lower-layer oils have the same oil source (from the Permian source rocks), and the upper-layer oils also have the same source (mainly from the Jurassic source rocks). In Chepaizi, the hydrocarbons stored in reservoirs originated mainly from Jurassic and Permian rocks. However, Cretaceous source rocks also have some contribution to oil-gas supply for oil pools. The oil-source division results based on nitrogen compounds (carbazoles) are completely identical with those by common geochemical indices, indicating that carbazoles are of importance in oil type division and oil-source correlation.

Evidence of Oil Sources and Migration in Triassic-Jurassic Reservoirs in the South Tianhuan Depression of the Ordos Basin, China Based on Analysis of Biomarkers and Nitrogen-Bearing Compounds

The Ordos Basin is an important intracontinental sedimentary basin in western China for its abundant Mesozoic crude oil resources. The southern part of the Tianhuan Depression is located in the southwestern marginal area of this Basin, in which the Jurassic and Triassic Chang-3 are the main oil-bearing strata. Currently, no consensus has been reached regarding oil source and oil migration in the area, and an assessment of oil accumulation patterns is thus challenging. In this paper, the oil source, migration direction, charging site and migration pathways are investigated through analysis of pyrrolic nitrogen compounds and hydrocarbon biomarkers. Oil source correlations show that the oils trapped in the Jurassic and Chang-3 reservoirs were derived from the Triassic Chang-7 source rocks. The Jurassic and Chang-3 crude oils both underwent distinct vertical migration from deep to shallow strata, indicating that the oils generated by Chang-7 source rocks may have migrated upward to the shallower Chang-3 and Jurassic strata under abnormally high pressures, to accumulate along the sand bodies of the ancient rivers and the unconformity surface. The charging direction of the Jurassic and Chang-3 crude oils is primarily derived from Mubo, Chenhao, and Shangliyuan, which are located northeast of the southern Tianhuan Depression, with oils moving toward the west, southwest, and south. The results show that an integration of biomarker and nitrogen-bearing compound analyses can provide useful information about oil source, migration, and accumulation.

Geochemical characteristics of crude oil and oil-source correlation of the Paleogene "Red Bed" in the south slope of the Dongying Depression, Bohai Bay Basin, China

The Paleogene Red Bed in the South Slope of the Dongying Depression is a set of red clastic sediments of the lower section of 4th member of the Shahejie Formation and the Kongdian Formation, and is the alternative strata for oil and gas exploration in this area. The Red Bed crude oil of the South Slope of the Dongying Depression is divided into four types according to the assemblage characteristics of biomarkers from 17 oil sand samples and 3 crude oil samples. The Type I crude oil is generated from the source rocks of the lower section of 3rd member of the Shahejie Formation, with low phytane and gammacerane concentration, Ph/n-C18<0.6, relatively high ratios of Ts/(Ts+Tm) (>0.38) and hopanes/steranes (>3.4), high diasteranes concentration, and low C35 homohopane index. The Type II crude oil could be derived from the source rocks of the upper section of 4th member of the Shahejie Formation with abnormal high phytane and gammacerane concentration, Ph/n-C181, relatively low ratios of Ts/(Ts+Tm) (<0.38) and hopanes/steranes (<2.5), low diasteranes concentration, and high C35 homohopane index. The Type III crude oil is mixed oil consisting of the Type II crude oil charged in the early stage and the Type I crude oil charged in the late stage and mainly of the Type I crude oil. The Type IV crude oil is characterized by C29 sterane predominance and this feature has not been found in the crude oils and source rocks of the study area, therefore this type of oil is identified as a mixture of the Type II crude oil and an oil type from a shallow lake-swamp facies. The fault throw of oil transporting faults controls the distribution of crude oils with different genetic types, and the upthrown side of the faults is the favorable zone for the Red Bed oil and gas accumulation.

Study of sedimentary sequence cycles by well-seismic calibration

In order to solve the problems of the fine division of sedimentary sequence cycles and their change in two-dimensional space as well as lateral extension contrast, we developed a method of wavelet depth-frequency analysis. The single signal and composite signal of different Milankovitch cycles are obtained by numerical simulation. The simulated composite signal can be separated into single signals of a single frequency cycle. We also develop a well-seismic calibration insertion technology which helps to realize the calibration from the spectrum characteristics of a single well to the seismic profile. And then we determine the change and distribution characteristics of spectrum cycles in the two-dimensional space. It points out the direction in determining the variations of the regional sedimentary sequence cycles, underground strata structure and the contact relationship.

Estimation on organic carbon content of source rocks by logging evaluation method as exemplified by those of the 4th and 3rd members of the Shahejie Formation in western sag of the Liaohe Oilfield

One of the most important tasks of evaluating natural resources of petroliferous basins is to determine the organic matter abundance of source rocks in the basin. The usual method for assessing the organic carbon content of source rocks is based on laboratory analyses. There is a deviation in calculating organic carbon content due to the heterogeneous distribution of organic matter and the artificial factors when sampling. According to the continuous characteristics of information logging, the conventional logging curves (mainly acoustics and resistivity, etc.) were calibrated with the organic carbon experimental data of cores, cuttings or sidewall cores. The organic carbon content of source rocks of the 4th (Es4) and 3rd (Es3) members of the Shahejie Formation in western sag in the Liaohe depression was estimated directly by a great amount of continuous data including resistivity and acoustic logging, etc. Comparison between the results from computer processing and lab analysis of logging data shows that the organic carbon contents derived from the computer processing of logging data have the same reliability and accuracy as the lab analysis results. The present data show that this method is suitable to evaluate the source rocks of western sag in the Liaohe depression and has great potential in evaluating natural resources of sedimentary basins in the future. On the basis of logging data of source rocks, experimental data and existing geochemical analyses of the Liaohe Oilfield, the corresponding total organic carbon (TOC) isograms of source rocks were plotted. The source rocks of Es4 and Es3 of the Shahejie Formation are thought to be beneficial to hydrocarbon accumulation due to the high TOC.

Research on stratigraphic division and sand bed correlation based on “three instantaneous” attribute spectra of logging data

The stratigraphic division and sand bed correlation are an important and difficult work for exploration and development of oil-gas field. The application of “three instantaneous” attribute spectrum of logging data on the research of stratigraphic division and sand bed correlation is carried out first time by the authors. In the method, mainly Hilbert-Huang transformation was used to extract the three attributes from logging data (natural gamma ray, Acoustic, and so on), i.e. “instantaneous frequency”, “instantaneous amplitude” and “instantaneous phase”, and then the technology of high-resolution profile reconstruction was adopted to reconstruct the “three instantaneous” attribute spectrum, to reveal the geological response characteristics on each depth of the internal formation, and to clearly reflect the variation characteristics of geological phenomena in the formation. The sequence boundary is scaled according to the alternating change of “three instantaneous” attribute energy spectrum, the migration cycle changes and distribution characteristics of attribute spectrum energy cluster of different frequency components are used to scale the stratigraphic units of different sizes and the response characteristics of sand and mud, and then the oscillation change trend and distribution information of “three instantaneous” attribute spectra energy cluster from low frequency to high frequency is utilized to do the division and correlation of formation and sand body, and to depict geological characteristics of formation internal. From the change and distribution of attribute spectrum energy cluster, the boundary of different sequence grade and distribution of sand bed in stratigraphic sequence can be clearly found. Then, sequence division and single sand body identification of all levels are realized. The single sand body isochronous correlation is compared under the control of all levels sequence division. The research results of this paper promote sedimentary reservoir bed and sequence stratigraphy development toward quantification and digitalization, and provide a new method for geological studies.

Tight sandstone gas accumulation mechanism and development models

AbstractTight sandstone gas serves as an important unconventional hydrocarbon resource, and outstanding results have been obtained through its discovery both in China and abroad given its great resource potential. However, heated debates and gaps still remain regarding classification standards of tight sandstone gas, and critical controlling factors, accumulation mechanisms, and development modes of tight sandstone reservoirs are not determined . Tight sandstone gas reservoirs in China are generally characterized by tight strata, widespread distribution areas, coal strata supplying gas, complex gas–water relations, and abnormally low gas reservoir pressure. Water and gas reversal patterns have been detected via glass tube and quartz sand modeling, and the presence of critical geological conditions without buoyancy-driven mechanisms can thus be assumed. According to the timing of gas charging and reservoir tightening phases, the following three tight sandstone gas reservoir types have been identified: (a) “accumulation–densification” (AD), or the conventional tight type, (b) “densification–accumulation” (DA), or the deep tight type, and (c) the composite tight type. For the AD type, gas charging occurs prior to reservoir densification, accumulating in higher positions under buoyancy-controlled mechanisms with critical controlling factors such as source kitchens (S), regional overlaying cap rocks (C), gas reservoirs, (D) and low fluid potential areas (P). For the DA type, reservoir densification prior to the gas charging period (GCP) leads to accumulation in depressions and slopes largely due to hydrocarbon expansive forces without buoyancy, and critical controlling factors are effective source rocks (S), widely distributed reservoirs (D), stable tectonic settings (W) and universal densification of reservoirs (L). The composite type includes features of the AD type and DA type, and before and after reservoir densification period (RDP), gas charging and accumulation is controlled by early buoyancy and later molecular expansive force respectively. It is widely distributed in anticlinal zones, deep sag areas and slopes, and is controlled by source kitchens (S), reservoirs (D), cap rocks (C), stable tectonic settings (W), low fluid potential areas (P), and universal reservoir densification (L). Tight gas resources with great resource potential are widely distributed worldwide, and tight gas in China that presents advantageous reservoir-forming conditions is primarily found in the Ordos, Sichuan, Tarim, Junggar, and Turpan-Hami basins of central-western China. Tight gas has served as the primary impetus for global unconventional natural gas exploration and production under existing technical conditions.

Charging History of Crude Oils in the Slope Belt of the Southern Junggar Foreland Basin, NW China: A Case Study from the East Edge of Chepaizi High

The east edge of Chepaizi High belongs to the western slope belt of the southern Junggar foreland basin, NW China. Based on the features of oil source and migration pathway, this study rebuilt the charging history of crude oils related to the slope belt reservoir forming in the east edge of Chepaizi High, comprehensively through the methods of fluid inclusion microscopic observation, microthermometry and microbeam fluorescence spectra. The results show that there are mainly 4 periods of oil charging including the Middle Jurassic to the Middle Cretaceous (the 1st period), the Middle-Late Cretaceous to the Early Paleogene (the 2nd period), the Early Neogene (the 3rd period) and the Middle-Late Neogene to the Quaternary (the 4th period). The 1st and 2nd period oils both generated from the Permian Wuerhe source rock charged into the Mesozoic reservoirs. The 1st period oil is low-mature to mature with long time and large-scale volume of oil charging, while the 2nd period oil was mature to highly mature with short time and small-scale volume of oil charging. Partial 1st period oil suffered biodegradation because of poor preservation conditions resulted from tectonic uplift in the Late Jurassic. In the Early Neogene, biodegraded oil gradually migrated into the shallow reservoirs and formed the secondary heavy oil reservoirs as the 3rd charging period. The 4th period oil was mainly from the Jurassic source rock. Through the migration pathways composited by sand bodies, faults and unconformities, the oil migrated into the Neogene Shawan reservoirs and formed the light oil reservoirs. Meanwhile, the oil was overprinted by some low maturity Cretaceous organic matter during upward migration, and some oil from the Jurassic source rock got mixed with the earlier charging heavy oils. For different tectonic evolution stages, there existed distinct oil charging characteristics in the study area.