Kongdian Formation Research Articles

Occurrence Characteristics and Controlling Factors of Shale Oil in Member 2 of Kongdian Formation in Cangdong Sag, Bohai Bay Basin, China

Occurrence Characteristics and Controlling Factors of Shale Oil in Member 2 of Kongdian Formation in Cangdong Sag, Bohai Bay Basin, China

Quantitative Evaluation of Pore Structures within Different Laminae ： a Case Study of Lacustrine Shales from the Second Member of Kongdian Formation in Cangdong Sag, Bohai Bay Basin, China

Quantitative Evaluation of Pore Structures within Different Laminae ： a Case Study of Lacustrine Shales from the Second Member of Kongdian Formation in Cangdong Sag, Bohai Bay Basin, China

Examining and applying the theory of “exploring petroleum inside source kitchens” for continental shale oil: A case study from the Kong 2 member of the Cangdong sag in the Bohai Bay Basin, China

Continental shale oil to be studied urgently is an important unconventional “source rock oil and gas” and key domain of “exploring petroleum inside source kitchens”. Based on the “source rock oil and gas” theory and the thought of “exploring petroleum inside source kitchens”, this paper established the ”six determination” exploration workflow for continental shale oil (i.e., determining the background, source rock, reservoir, play, resources, and sweet spot), which geological characteristics and exploration workflow are significantly different from conventional hydrocarbon. A case study was conducted by the workflow in the Kong 2 member (Ek2) of the Cangdong sag in the Bohai Bay Basin, China. Semi-deep to deep lacustrine facies belts and many gentle slope structures (belts) were widespread during Kongdian Formation depositing, thus providing favorable conditions for the formation and large-area accumulation of shale oil. Ek2 developed broader-distributed, and thick-bedded dark mudstone and oil shale, which exhibit high TOC, a good kerogen type, and great hydrocarbon-generating potential. Siltstone to fine sandstone and mud to micritic dolomite reservoirs were found within the low-dip slopes and central zone, which are thick, overlapped, abundant high-angle fractures, and exhibit low porosity and ultra-low permeability. Sandstone-type and dolomite-type shale oil of Ek2 characterized source–reservoir integration and five types of sources–reservoir configurations. They mainly overlapped at the front end of the braided river delta in the slope zone and in the sag center, where are superimposed vertically, and are connected horizontally with conventional structural–lithological oil pools in the up-dip slope. The median geological resources of the two classes shale oil were 5.26×108/t, with the remaining resources accounting for 90% of the total and moderate buried depths, thus the exploration potential is great. Several special exploration wells have obtained industrial oil flow. There are two types of sweet spots: low permeability sandstone layers and interbedded layers of oil shale and dolomite. A few favorable sweet spots were identified as targets for subsequent exploration. Then, three aspects of thinking and recommendations to solve the next exploration direction and technical problems were proposed, which will provide important reference and theoretical guidance for the further exploration of the same type of shale system in China and the world.

Lamination Texture and Its Effects on Reservoir and Geochemical Properties of the Palaeogene Kongdian Formation in the Cangdong Sag, Bohai Bay Basin, China

The characteristics of laminae are critical to lacustrine shale strata. They are the keys to the quality of source rocks and reservoirs, as well as engineering operations in shale plays. This study uses organic geochemistry, thin section identification, X-ray diffraction, field emission scanning electron microscopy, and other analytical methods, to reveal the detailed lamination texture and vertical distribution of laminae in the second Member of the Kongdian Formation in Cangdong Sag. The principal results are as follows: (1) A classification of laminae is proposed to characterize reservoir and geochemical properties. The five types of laminae are as follows: feldspar-quartz laminae (FQL), clay laminae (CLL), carbonate laminae (CAL), organic matter laminae (OML), and bioclastic laminae (BCL). There are also four significant lamina combinations (with the increasing TOC values): FQL-CLL combination, FQL-CLL-BCL combination, FQL-CLL-OML combination, and FQL-CAL-CLL-OML combination; (2) differences between laminae occur because of the variability in pore types and structures. There appears to be a greater abundance of intercrystalline pores of clay minerals in the FQL, CAL, BCL, and OML, and well-developed organic pores in the CAL and CLL, and the counterparts of intragranular pores of bioclastic material in the BCL. This detailed characterization provides the following comparative quantification of the thin section porosity of laminae in the second Member of the Kongdian Formation can be differentiated: CAL > FQL > OML > BCL > CLL; (3) differentiation between vertical distributions of laminae is carried out in a single well. The FQL and CLL are widely distributed in all the samples, while the BCL is concentrated in the upper part of the second Member of the Kongdian Formation, and CAL is concentrated in the lower part. This detailed classification method, using geochemical analysis and vertical distribution descriptions, offers a detailed understanding of lamination texture and its effects on reservoir and geochemical properties, which will provide a scientific guidance and technical support to better estimate reservoir quality and to identify new sweet spots in the second Member of the Kongdian Formation in the Cangdong Sag.

Characteristics and Control Mechanism of Lacustrine Shale Oil Reservoir in the Member 2 of Kongdian Formation in Cangdong Sag, Bohai Bay Basin, China

The significance of lacustrine shale oil has gradually become prominent. Lacustrine shale has complex lithologies, and their reservoir properties are quite various. The multi-scale pore structure of shale controls the law of shale oil enrichment. Typical lacustrine shale developed in the Member 2 of Kongdian Formation in Cangdong sag, Bohai Bay Basin. The lithofacies and multi-scale storage space of this lacustrine shale have been systematically studied. 1. The mineral composition is quite different, and the lithofacies can be summarized into siliceous, carbonate and mixed types. The rock structure can be summarized into laminated, layered, and massive types. 2. The pores are diverse and multi-scale. Interparticle pores contribute the main storage space, especially the interparticle pores of quartz and dolomite. 3. The physical properties of the massive shales is relatively inferior to those of layered and laminatedtypes, and it presents the characteristics of " laminated >layered > massive ". The developed laminae can significantly improve the space and seepage capacity of lacustrine shale. 4. Clay minerals provide the main nano-scale storage space, but they are often filled in pores and reduces the shale brittleness, which have destruction effects.

Pore Structure and Fractal Characteristics of Organic-Rich Lacustrine Shales of the Kongdian Formation, Cangdong Sag, Bohai Bay Basin

In order to examine the pore structure and reveal the fractal geometric nature of shales, a series of laboratory experiments were conducted on lacustrine shale samples cored from the Kongdian Formation. Based on the low temperature nitrogen adsorption, fluorescent thin section and field emission scanning electronic microscope, a comprehensive pore structure classification and evaluation were conducted on shale samples. Fractal dimensions D1 and D2 (with relative pressure of 0–0.45 and 0.45–1.00, respectively) were obtained from the nitrogen adsorption data using the fractal Frenkel-Halsey-Hill (FHH) method. With additional means of X-ray diffraction analysis, total organic carbon content analysis and thermal maturity analysis, the relationships between pore structure parameters, fractal dimensions, TOC content and mineral composition are presented and discussed in this paper. The results show that interparticle pores and microfractures are predominant, whereas organic matter pores are rarely found. The pore morphology is primarily featured with wide-open ends and slit-shaped structures. In terms of pore scale, mesopores and macropores are predominant. The value of fractal dimension D1 representing small pores ranges from 2.0173 to 2.4642 with an average of 2.1735. The value of D2 which represents large pores ranges from 2.3616 to 2.5981 with an average of 2.4960. These low numbers are an indication of few pore types and relatively low heterogeneity. In addition, smaller D1 values reveal that large pores have more complicated spatial structures than smaller ones. The results of correlation analysis show that: 1) D2 is correlated positively with specific surface area but negatively with average pore diameter; 2) D1 and D2 literally show no obvious relationship with mineral composition, TOC content or vitrinite reflectance (Ro); 3) both total Barrett-Joyner-Halenda (BJH) volume and specific surface area show a positive relationship with dolomite content and a negative relationship with felsic minerals content. These results demonstrate that the pore types are relatively few and dominated by mesopores, and the content of brittle minerals such as dolomite and felsic minerals control the pore structure development whilst organic matter and clay minerals have less influence due to low thermal maturity and abundance of clay minerals.

Provenance and sedimentary environment of the Ek2 shale in the Cangdong Sag, the central Bohai Bay Basin, China

The second member of the Kongdian Formation (usually abbreviated as the E k2 shale) is one of the most significant exploring targets for shale oil at the Cangdong Sag of the central Bohai Bay Basin. It consists of siliceous shale, mixed shale, and calcareous shale. To better understand why organic matter accumulated in the E k2 shale, we have analyzed major and trace elemental compositions to reconstruct the provenance and sedimentary environment. Tectonic discriminatory diagrams suggest that the tectonic setting of the parental rocks for the E k2 shale belonged to the Continental Island Arc. The distribution patterns of trace elements and rare earth elements + yttrium (REEs + Y) are close to the intermediate igneous rock. The ratios of Al2O3/TiO2 ranging from 21.41 to 27.59 with a mean value of 23.93 also demonstrate a parental rock of the intermediate igneous rock. Siliceous and mixed shales indicate K2O/Al2O3 of 0.17–0.29, chemical index of weathering of 28.79–97.79, plagioclase index of alteration of 38.24–95.57, and chemical index of alteration of 40.29–80.23. These weathering proxies denote that the E k2 shale underwent a low weathering degree in an arid climate and a high weathering degree in a semiarid climate. The V/(V + Ni) ratios and pyrite framboids indicate an anoxic sedimentary condition. The δ18O values of carbonate minerals in the E k2 shale range from −9.8‰ to 0.7‰, and they are positively correlated to the δ13C values. The Sr/Ba ratios, δ18O, and chemical mineral associations indicate that siliceous and mixed shales were deposited in a fresh to brackish anoxic water column under a semiarid climate. Whereas calcareous shale was deposited in a saline to hypersaline anoxic water column under an arid climate.

Magmatic hydrothermal fluid genesis of zeolite in the Paleogene Kongdian Formation of Zaoyuan oilfield, Bohai Bay Basin, China

Electronic probe, fluid inclusion homogenization temperature, Raman spectroscopy and laser ablation inductively coupled plasma mass spectrometry were utilized to identify the hydrothermal fluid-rock interactions in the second member of the Paleogene Kongdian Formation of Zaoyuan oilfield in Bohai Bay Basin (Kong 2 Member for short) of Well Z56 to find out the relationship between zeolite and hydrothermal fluid. The experimental results show that: (1) Pyrobitumen coexists with hydrothermal fluid characteristic minerals such as chlorite, barite, chalcopyrite, pyrite, natrolite and analcime in mudstone fractures. (2) The temperatures calculated from laser Raman spectrum of pyrobitumen, from the chlorite geothermometer and from measured homogenization temperature of natrolite inclusions are 324–354 °C, 124–166 °C and 89–196 °C, respectively; although vary widely, all the temperatures are obviously higher than the normal geothermal temperature. (3) The positive Eu anomaly of chlorite and barite, and the similar distribution pattern in rare earth elements between natrolite and basalt indicate they are from magmatic hydrothermal fluid. Moreover, drilling data shows that the Kong 2 Member in Well Z56 has several sets of basalt interlayers, suggesting there was geologic base of magmatic hydrothermal fluid activity. The magmatic hydrothermal fluid-rock interaction may be one of the reasons for the abnormal enrichment of zeolite in Kong 2 Member of the Cangdong Sag.

A distinct oil group in the Dongying Depression, Bohai Bay Basin, China: New insights from norcholestane and triaromatic steroid analyses

The lower third and upper fourth members of the Eocene Shahejie Formation (lower Es3 and upper Es4) are known as two sets of effective source rocks in the Dongying Depression, Bohai Bay Basin. In this study, the saturated and aromatic steroids of 20 crude oil samples from the Dongying Depression were systematically investigated using gas chromatography–mass spectrometry (GC–MS) and GC–MS metastable reaction monitoring. Based on the distribution patterns of the steranes, norcholestanes, and triaromatic steroids, the Group I and II oils were clearly identified and shown to be derived from the lower Es3 and upper Es4 source rocks, respectively. Significantly, it was discovered that three oil samples collected from the Eocene Kongdian Formation and the Ordovician reservoirs exhibit completely different steroid and isotopic compositions from those of the Group I and II oils. These oils, as a distinct oil group (Group III), have the characteristics of abnormally high concentrations of C29 steranes, 24-nordiacholestane, 27-nordiacholestane, C28 triaromatic steroids, C27 4α-methyltriaromatic steroids, and C29 4α-methyl-24-ethyltriaromatic steroids, and low concentrations of 24-norcholestane, 27-norcholestane and triaromatic dinosteroids, and 13C depletion. Although the related source rock of Group III oils needs to be further clarified in the study area, the 13C depletion, abundant C29 steranes, and low 24-norcholestane content may suggest greater bacteria/algae inputs (e.g., green algae rather than dinoflagellates) than both the lower Es3 and upper Es4 source rocks. Identification of the Group III oils indicates that the norcholestanes and triaromatic steroids may be more effective molecular fossils for the classification of crude oils in complicated petroleum exploration fields.

Sweet spot evaluation and exploration practice of lacustrine shale oil of the second member of Kongdian Formation in Cangdong sag, Bohai Bay Basin

Based on core, thin section, X-ray diffraction, rock pyrolysis, CT scanning, nuclear magnetic resonance and oil testing data, the macro and micro components, sedimentary structure characteristics, of Paleogene Kong 2 Member in Cangdong sag of Huanghua depression and evaluation standard and method of shale oil reservoir were studied to sort out the best shale sections for shale oil horizontal wells. According to the dominant rock type, rhythmic structure and logging curve characteristics, four types of shale lithofacies were identified, namely, thin-layered dolomitic shale, lamellar mixed shale, lamellar felsic shale, and bedded dolomitic shale, and the Kong 21 sub-member was divided into four quasi-sequences, PS1 to PS4. The PS1 shale has a porosity higher than 6%, clay content of less than 20%, and S1 of less than 4 mg/g; the PS2 shale has well-developed laminar structure, larger pore and throat size, better connectivity of pores and throats, high contents of TOC and movable hydrocarbon, S1 of over 4mg/g, clay content of less than 20%, and porosity of more than 4%; PS3 shale has S1 value higher than 6 mg/g and clay content of 20% - 30%, and porosity of less than 4%; and PS4 shale has lower TOC content and low oil content. Shale oil reservoir classification criterion based on five parameters, free hydrocarbon content S1, shale rhythmic structure, clay content, TOC and porosity, was established. The evaluation method of shale oil sweet spot by using the weighted five parameters, and the evaluation index EI were proposed. Through comprehensive analysis, it is concluded that PS2 is best in quality and thus the dual geological and engineering sweet spot of shale oil, PS3 and PS1 come next, the former is more geologic sweet spot, the latter more engineering sweet spot, and PS4 is the poorest. Several vertical and horizontal wells drilled in the PS2 and PS3 sweet spots obtained high oil production. Among them, Well 1701H has produced stably for 623 days, with cumulative production of over 10000 tons, showing bright exploration prospects of Kong 2 Member shale oil.

Paleoenvironment evolution of the lacustrine organic-rich shales in the second member of Kongdian Formation of Cangdong Sag, Bohai Bay Basin, China: Implications for organic matter accumulation

The second member of Kongdian Formation (Ek2) of Cangdong Sag represents an excellent hydrocarbon source rocks interval target for crude oil and shale oil production in the Bohai Bay Basin. To reconstruct the palaeoenvironmental conditions and reveal the mechanisms of organic matter enrichment of the Ek2 shales, a series of analyses (total organic carbon (TOC), Rock-Eval pyrolysis, major and trace element analyses, and X-ray diffraction (XRD)) were performed on samples collected from the borehole of Cangdong Sag. The Ek2 consists of fine-grained rocks deposited in a lacustrine environment, and it can be further subdivided into four members (Ek24 to Ek21). The results show that the Ek2 contains a high abundance of thermally mature Type I and II kerogen, and most of the samples show good to excellent generative potential. The mineralogical compositions, dominated by quartz, feldspar, and dolomite, with subordinated calcite, clay minerals, and analcime. Specific major and trace elements were employed to analyze the changes in paleoclimate conditions, lake water chemistry, and the detrital influx of Ek2. The vertical changes of paleoenvironmental indicators suggest that during the sedimentary period of Ek2, the climate changed from semi-humid to arid, with the input of terrigenous detrital gradually decreased, and the salinity of the water increased. The relationship among the primary productivity, redox conditions, detrital influx indicators and TOC content suggests that primary productivity was the most critical factor in controlling organic enrichment, and the input of terrigenous debris promotes the increase in productivity. The high primary productivity, together with anoxic bottom water conditions, resulted in the greatest enrichment of organic matter in the lower part of Ek2 shale (Ek23). The upper part (Ek21) is mainly composed of moderate-TOC calcareous shale, which is deposited under arid paleoclimate, with low detrital input, moderate productivity, and favorable preservation conditions.

Pore Structure Characterization and Reservoir Quality Evaluation of Analcite-Rich Shale Oil Reservoir from the Bohai Bay Basin

Pore characteristics and shale oil reservoir quality strongly influences the petroleum occurrence, enrichment, and economic production. In this work, to characterize the pore system and evaluate the reservoir quality of analcite-rich shale oil reservoir, a total of 33 shale samples from the second member of the Paleogcene Kongdian Formation (Ek₂) of Bohai Bay Basin were examined using a suite of techniques, including total organic carbon (TOC) content, X-ray diffraction (XRD), field emission SEM, gas (CO₂ and N₂) physisorption, and mercury injection porosimetry (MIP). Six typical shale lithofacies are identified, and their pore characteristics are compared to reveal the factors affecting shale reservoir quality. The mineral matrix-related pore (e.g., intergranular and intragranular pores) is the dominant pore type in different shale lithofacies, with few organic matter (OM)-hosted pores being observed. Analcite-rich siliceous shale exhibits the largest specific surface area (SSA) and micropore volume, while the smallest is observed in analcite-rich mixed shale lithofacies. In contrast, the volume and SSA values of mesopores for siliceous shale are the largest, while mixed shale has the smallest values. Multipermeabilities of five connected pore networks were determined using the Katz-Thompson equation, and the permeability of microfractures or interlayer bedding cracks is at the micro-Darcy scale, as reflected by the first two inflection points. Permeability values for the other three inflection points reflect the matrix pore system within a nano-Darcy range. Siliceous shale has the high matrix permeability and good pore connectivity, which is favorable for fluid flow. Authigenic analcite mineral has different effects on pore development and reservoir quality depending on the shale mineral composition and diagenesis processes. Some primary pores were lost during the filling of analcite mineral in the early diagenesis period, but some intergranular pores were also protected by the rigid framework formed by analcite minerals. In addition, some secondary dissolution pores are generated during the reaction between analcite and organic acids discharged from the source rocks within the oil generation window, and the reservoir porosity and permeability can be greatly improved. Except for analcite, the TOC content is also an important factor controlling reservoir quality.

Quantitative Method for Evaluating Shale Oil Resources Based on Movable Oil Content

The light hydrocarbon content ( S 1 ) of shale oil is lost in aboveground experimental measurements, which do not accurately reflect actual underground light hydrocarbon content and cannot meet the demands of resource quantity calculation. Based on field and laboratory experimental data from the second member of the Kongdian Formation in the Cangdong Sag, Bohai Bay Basin, the retained oil and movable oil contents in shale were calculated using a mathematical formula, and the total resources were quantified. The correction coefficient of S 1 from pyrolysis and the adsorption of oil by the total organic carbon (TOC) were determined to be 1.25 and 100 mg/g, respectively. The calculated parameter movable oil content ( S movable ) and the corresponding calculation formula of S 1 are proposed. The lower limit of S 1 is 100 mg HC/g rock, the TOC content is 4 wt % and 6 wt %, and the corrected movable oil content of 3 mg/g and 6 mg/g, respectively, will be of great significance in shale oil geology and engineering. The optimal geological and engineering settings are divided into three categories and eight subcategories on the basis of these findings. Methods for evaluating total, movable, and recoverable oil resources from shale are discussed, which elucidate a new method for quantitative evaluation and ranking of shale oil resources. This approach is suitable for application in other shale oil exploration and development areas globally.

Microscale comprehensive evaluation of continental shale oil recoverability

Abstract This paper targets the shale oil reservoirs of middle to high maturity in four major basins of China, including the Permian Lucaogou Formation of the Jimsar Sag in the Junggar Basin, the Chang 73 Member of the Triassic Yanchang Formation in the Longdong area of the Ordos Basin, the Kong 2 Member of the Paleogene Kongdian Formation in Cangdong Sag of the Bohai Bay Basin, and the Qing 1 Member of the Cretaceous Qingshankou Formation in Changling Sag of the Songliao Basin. The key parameters of the shale oil reservoirs in the four basins, such as reservoirs effectiveness, oil content, crude oil movability, and fracability, have been revealed under identical experimental conditions using the same evaluation technical system, on the basis of technique development and integrated application of multi-scale spatial distribution depiction, effective connectivity calculation, movable oil assessment based on the charging effect, and simulation of fracture propagation during reservoir stimulation. This research overcomes insufficient resolutions of conventional analysis approaches and difficulties in quantitative evaluation, develops the evaluation method for resource recoverability of different types of shale oil, and gains insights into different types of shale oil via comparison. The results of experiments and comparative analysis show that there are significant differences in the endowment of continental shale oil resources in the four major basins in China. Among them, the Lucaogou Formation in the Junggar Basin has more effective shale reservoirs, the Chang 73 sub-member of the Ordos Basin has a comparatively good proportion of movable oil and the Kong 2 Member of the Bohai Bay Basin has the best fracability. These results can provide references and basis for choosing development plans and engineering techniques.

Influence of two-phase extension on the fault network and its impact on hydrocarbon migration in the Linnan sag, Bohai Bay Basin, East China

Multiphase rifts produce fault networks formed by the non-coaxial faults, these networks evolve through the formation of new faults and pre-existing faults. Pre-existing faults have a great influence on the formation of new faults under later regional stress, resulting in a complex fault network. Here, abundant 3D seismic and log data are used to reveal the evolution of fault networks in the Linnan sag in southwestern East China, a complex basin that experienced multiple phases of extension during the Cenozoic and developed NE-SW-, ENE-WSW- and E-W-striking faults. (1) During deposition of the Kongdian formation (65-50 Ma), NE-SW-striking faults formed under regional NW extension. In contrast, the ENE-WSW-striking and E-W-striking faults are younger, as they show no impact on the Ek formation. (2) During deposition of the Shahejie 4 formation (50-42 Ma), faults of all orientations (NE-SW, ENE-WSW and E-W) were active. However, the pre-existing NE-SW-striking faults show dextral strike-slip characteristics. The ENE-WSW-striking faults in the central parts show shear properties. The minor faults controlled by the regional extension and local strike-slip faulting near the pre-existing NE-striking faults strike NE, and the E-W-striking faults are distributed far from the pre-existing fault. These phenomena all indicate that the stress field changed from NW-SE to N-S extension in this stage. (3) During deposition of the Shahejie 3 formation (42-38 Ma), all fault activity was the strongest, and NE-striking faults began to connect and control basin deposition. The ENE-WSW-striking faults became longer. The density of E-W-striking faults increased. (4) During deposition of the Shahejie 2-Dongying formations (38–23.5 Ma), all fault activity weakened, and the Linnan sag received sediment. These observations demonstrate that later extension with a different direction can form local stress near pre-existing faults and that faults with new strikes can enhance the geometric and kinematic complexity in the fault network in the late stage. This study provides a reference for the interpretation of other multiphase rift, where two-phase extension fault networks were controlled by regional and local stresses, the reactivated pre-exist faults and newly-formed faults coexist in non-coaxial extension. Additionally, such fault networks can have important controlling effects on the distribution of hydrocarbon accumulation.

Research on the Formation Geological Conditions and Development Stages of Source Rocks in Linnan sag

The study area is located in the Linnan subsag in the west of the Jiyang Depression of the Bohai Bay Basin. Its structural axis is in the NNE direction. It mainly develops four structural units: the Linnan slope zone, the Xiakou fault zone, the Linnan subsag zone and the central uplift zone. , The exploration area is about 1300 km2. The formation and evolution of the Linnan sag can be roughly divided into four stages: the Mesozoic fault depression, the slope of the Kongdian Formation-Upper Es4 deposition period, the fault depression during the Es3 Member-Dongying Formation deposition period, and the Neogene depression. During the formation of the sag, a complete Paleogene strata was deposited, which was divided from top to bottom into the Kongdian Formation, Shahejie Formation (which can be subdivided into four sections), Dongying Formation, Guantao Formation and Minghuazhen Formation , And formed multiple sets of source, reservoir, and cap rock assemblages. The Shahejie Formation is not only the most important source rock development interval in the Linnan Sag, but also an important reservoir rock and cap rock development interval. Various types of oil and gas reservoirs, especially lithological oil and gas reservoirs, are important exploration targets for increasing reserves and production in this area. The study of the geological conditions and development stages of the source rocks in the Linnan Sag is not only a basis for establishing a hydrocarbon accumulation model, but also a theoretical basis for oil and gas exploration. Therefore, it is very necessary to analyze the organic geochemical characteristics and oil and gas sources of the source rocks in the study area.

Pore structure typing and fractal characteristics of lacustrine shale from Kongdian Formation in East China

Nine as-received core samples were selected from four wells drilled in the lacustrine Kongdian Formation, which is a source rock located in Cangdong Sag, Bohai Bay Basin, China, and their geochemistry and pore structure characteristics were investigated by Rock-Eval pyrolysis, low-pressure nitrogen physisorption (LNP), mercury intrusion porosimetry (MIP), and spontaneous imbibition (SI) tests. Various models were employed to investigate the fractal characteristics of pore systems, and discuss the relationships between pore structure parameters and fractal dimension (Df). The results show that the samples can be divided into two groups (A and B) according to the different types of hysteresis loops from LNP isotherms, as well as geochemistry and pore structure parameters of samples. Negative relationship among BJH (Barrett-Joyner-Halenda) pore volume and S1 value, and obviously higher average pore diameter of Group B samples with significantly higher S1 values, indicate a blockage of residual hydrocarbons on small pores. According to the different Df−MIP values from MIP tests, the pore-throats of samples can be divided into four categories of A to D in the following sequence: microscopic (diameters less than 50 nm), small (50 nm-8 μm), medium (8 μm–20 μm) and large (greater than 20 μm) pore-throats/pores. The difference of Df from LNP and MIP tests is probably due to different sample sizes used and pore types assessed, while the difference of Df between MIP and SI tests is attributed to the mixed wettability of pores in shale samples. In addition, from the results of LNP tests with a measurable pore size of 2–200 nm, obvious positive relationships exist between Df−LNP−C (LNP test-derived Df value for Region C) and BJH pore volume, BET (Brunauer-Emmett-Teller) surface area and average pore size, as well as microscopic pore volume and percentage. The Df−LNP−C can be considered as an effective parameter to reflect the degree of development of microscopic pores in samples.

Geological characteristics and shale oil potential of the lacustrine immature to low mature shale oil system: a case study of the second member of Kongdian Formation in Cangdong Sag, Huanghua Depression, Bohai Bay Basin, China

ABSTRACT The Second Member of Kongdian Formation (Ek2) shale oil system is a set of thick organic-rich lacustrine shales with great shale oil resources potential. The shales are oil-prone with high organic matter abundance (samples with TOC > 2% accounting for 72.6%) and low maturity (the Ro of samples between 0.42% and 0.84%). The Ek2 shale system is dominated by shales, sandstones, and carbonate rocks, which can be subdivided into various-mixed lithologies. The shales are primarily composed of clay minerals, quartz, feldspar, calcite, dolomite, pyrite, siderite, and zeolite, with no obvious predominant minerals. The brittle mineral content is high with an average content of 73.86%, which is conducive to reservoir stimulation. Physical properties and oil saturations vary significantly among different lithologies. Tight sandstones (average porosity 8.66%, average permeability 2.207 × 10−3 μm2) and dolomites (average porosity 5.26%, average permeability 1.910 × 10−3 μm2) have better exploration potential than shales (average porosity 2.46%, average permeability 0.937 × 10−3 μm2). Considering Ro distribution, the slope zones are more favorable than Kongdian Structure Belt laterally and the lower intervals of the Ek2 shale are more potential for shale oil accumulation than the upper interval vertically.

Key exploration & development technologies and engineering practice of continental shale oil: A case study of Member 2 of Paleogene Kongdian Formation in Cangdong Sag, Bohai Bay Basin, East China

Abstract Based on detailed core description and systematic joint test data, enrichment laws of continental shale oil have been examined deeply. Key technologies such as the identification and quantitative evaluation method for sweet spot, precise design and tracking of horizontal well trajectory, and the low-cost horizontal well volume fracturing technology of the whole process “slick water + quartz sand” for continental shale oil have been formed. The research results show that the enrichment of pure continental shale oil of the Paleogene Kong 2 Member in Cangdong Sag is controlled by predominant fabric facies and cross-over effect of retained hydrocarbons jointly; and there are four modes of shale oil enrichment, i.e. laminar felsic, laminar mixed, thin-layer limy dolomitic, and thick-layer limy dolomitic shales. The identification and evaluation method for shale oil sweet spots can predict sweet spots accurately. The precise trajectory design for sweet spot layer and tracking-trajectory optimization while drilling by considering geological and engineering factors have been proved effective by field application, with drilling rate of sweet spots reaching 100% and drilling rate of type I sweet spots reaching over 75%. The whole process “slick water + quartz sand” low cost volume fracturing has been proved effective in creating multi-stage fracture network in the horizontal section, and improved productivity greatly. It can lower the comprehensive engineering cost by 26.4%.

Formation conditions and enrichment model of retained petroleum in lacustrine shale: A case study of the Paleogene in Huanghua depression, Bohai Bay Basin, China

Abstract Compared with marine facies shale strata, lacustrine shale strata are more complicated in geological conditions, and thus more difficult to explore and develop. To realize economic exploration and development of lacustrine shale oil, the geological regularities of accumulation and high yield of retained movable petroleum in shale should be understood first. In this work, taking the shale strata of Kong 2 Member and Sha 3 Member in the Paleogene of Huanghua depression in the Bohai Bay Basin as examples, based on the previous joint analysis results of over ten thousand core samples and the latest oil testing, production test and geochemical data of more than 30 horizontal wells, accumulation conditions and models of retained movable petroleum in lacustrine shale were studied comprehensively. The study shows that at moderate organic matter abundance (with TOC from 2% to 4%), shale strata have the best match between oil content and brittleness, and thus are rich in oil and good in fracability. Moderate ancient lake basin size and moderate sediment supply intensity are the internal factors leading to best coupling of organic matter abundance and brittle mineral content in the shale formation. Moderate thermal evolution maturity of Ro of 0.7%–1.0% (at burial depth of 3200 to 4300 m) is the interval where oil generation from thermal evolution and oil adsorption by kerogen in shale layers match best, and retained movable petroleum is high in proportion. Moderate diagenetic evolution stage (3200 to 4300 m in the middle diagenetic stage A) is conducive to the formation of a large number of dissolved pores and organic matter pores, which provide storage space for shale oil enrichment. Moderate development degree of natural fractures (without damaging the shale oil roof and floor sealing conditions) is conducive to the storage, seepage and preservation of shale oil. The research results have overthrown the general understanding that high organic matter abundance, high maturity, and high development degree of natural fractures are conducive to shale oil enrichment, and have guided the comprehensive evaluation of shale oil and gas sweet spots and well deployment in the second member of the Kongdian Formation in the Cangdong sag and the Shahejie Formation in the Qikou sag. Industrial development of the shale oil in Kong 2 Member of the Cangdong sag has made major breakthrough, and important signs of shale oil have been found in Sha 3 Member of the Qikou sag, demonstrating huge exploration potential of lacustrine shale oil.