•Meteoric water flushing may result in a net increase of porosity.•Authigenic clay minerals contain a large amount of microporosity.•At greater depths, secondary porosity may only represent porosity redistribution.The Chang7 and Chang6 sandstones in the southwestern Ordos basin are the typical tight oil sandstones in China. Pore textures and their genetic classes of secondary porosity were studied in detail using a combination of optical petrography, SEM and BSE. In order to understand generation process of secondary porosity better, the diagenetic history was reconstructed by a variety of methods, including thin section observation, SEM, BSE and fluid inclusion. Three main diagenetic events is considered to be most likely related to the generation of secondary porosity, these are: 1) meteoric water flushing; 2) formation of authigenic clay minerals; 3) leaching of feldspars, rock fragments and carbonate cements in deep burial sandstones. Particular attention was paid to the generation of secondary porosity and mass transformation in these diagenetic events. This study suggests only meteoric water flushing, which occurs at shallow depths, may result in a net increase of porosity because dissolved minerals can be removed out of sandstones in such an open system. However, below the zone of freshwater flow, dissolved minerals would precipitate as new diagenetic minerals at nearby pore spaces in sandstones, suggesting secondary porosity formed by dissolution only represents a local redistribution of the solids rather than a net increase of porosity.

•Sedimentary environment and diagenesis jointly lead to the tightness of reservoir.•Initial porosity loss due to compaction is greater than that due to cementation.•Tightening process of tight oil sand reservoir can be divided into four main stages.•Reservoirs were already tight prior to undergoing hydrocarbon accumulation.Upper Triassic Yanchang Formation in the Ordos Basin, northwest China, comprise the typical tight oil reservoirs, which have porosity less than 10% and permeability less than 1 mD. The western Jiyuan area (WJA), at the western margin of the Ordos Basin, has made significant progress in the tight oil exploration of the Yanchang Formation recently. Whereas the lack of recognition of tightening mechanism and process of reservoirs resulting in the misunderstanding of tight oil reservoirs prediction. This study focuses on the petrology, diagenetic processes, as well as forming mechanism and process of the Chang 8 (Ch-8) tight oil reservoirs located in the WJA. Four conclusions were reached. First, Ch-8 tight reservoirs, which are mainly composed of lithic arkose and feldspathic litharenite, experienced diagenesis, such as mechanical and chemical compaction, cementation, and dissolution. The reservoirs are at the late period of the mesogenetic A stage. Second, the tightness of the Ch-8 reservoirs is due to both sedimentary and diagenetic factors. The sedimentary environment where sand bodies are close to the provenance that has a rapidly increasing lacustrine level provides conditions for reservoir tightness. The initial porosity rapidly decreases due to compaction and cementation which produce porosity loss rates of 49.42% and 40.87%, respectively. By contrast, the effect of dissolution is limited, as it only increases porosity by 4.48%. Third, the tightening process of the Ch-8 reservoirs comprise four stages: compaction causes pores to reduce rapidly; followed by early cementation, which causes pores to reduce; dissolution, which causes pores to increase slightly; and late cementation, which causes tightness. Finally, it is likely that the Ch-8 reservoirs were already tight prior to hydrocarbon accumulation, thus demonstrating that they represent typical tight oil reservoirs.Download high-res image (476KB)Download full-size image

•Five episodes of brine inclusions and three episodes of oil inclusions occurred.•Three charging episodes (142–139 Ma, 128–122 Ma and 114–106 Ma) were identified.•PDBSR provided the main expulsion and charging force for crude oil.•The lower limit of the PDBSR is 3.97 MPa.This study examines the organic geochemistry, physics and accumulation dynamics of the Chang 7 tight oil reservoirs in the Yanchang Formation in the Ordos Basin. We analyse the episodes, timing, and forces of hydrocarbon charging and ascertain the charging process of the lacustrine tight oil in the Chang 7 oil reservoirs by using petrography, micro-beam fluorescence spectra, micro-thermometry, and trapping pressure simulations of fluid inclusions from these reservoir beds. Three conclusions are reached. First, five episodes of brine inclusions and three episodes of hydrocarbon inclusions occurred in the Chang 7 tight reservoir beds. The hydrocarbon inclusions are mainly distributed in fractures that cut across and within quartz grains and have blue-green, green, and yellow-green fluorescence colours. Second, the peak wavelengths, QF535, and Q650/500 of the micro-beam fluorescence spectra indicate three charging episodes of tight oil. According to the burial and geothermal histories, the charging timing were 142–110 Ma, 128–122 Ma and 114–106 Ma, which correspond to the Early Cretaceous. The timing approximately match the main generation and expulsion timing (140–110 Ma) of the Chang 7 source rocks. Third, the pressure differences between the source rocks and reservoir rocks, whose lower limit was 3.97 MPa, served as the primary expulsion and charging forces for crude oil during the charging periods and guaranteed the occurrence of non-buoyancy migration and accumulation.

•Sterane indices indicate low maturity for Neogene light oils of the study area.•Aromatic maturity indices indicate high maturity on the contrary.•The equivalent vitrinite reflectance of these light oils could reach ∼0.9–1.2%.•Overprinting of less mature oils is the main cause leading to this contradiction.•Phase fractionation may be a secondary cause of abnormal sterane maturity indices.The saturated and aromatic hydrocarbon maturity characteristics of crude oils from the eastern Chepaizi High in the Junggar Basin of northwest China were analyzed in detail. The results show that sterane maturity parameters for light oils from the Neogene Shawan Formation differ from other maturity parameters. Sterane αααC29 20S/(20S + 20R) and C29 ββ/(ββ + αα) ratios indicate low maturity, while aromatic hydrocarbons indicate high maturity. We infer that light oils from the Shawan Formation have higher maturity than Cretaceous and Jurassic crude oils and were generated at equivalent vitrinite reflectance of ∼0.9–1.2%. It can be concluded that the Shawan light oils originated mainly from deeply buried, highly mature Jurassic source rocks, southeast of Sikeshu Sag, southwest of Shawan Sag, and west of Huomatu Anticline Belt. During upward migration, crude oils generated from Jurassic source rock dissolved less mature Cretaceous bitumen resulting in a mixture with apparently conflicting parameters. Phase fractionation during migration also could have contributed to the anomalously low sterane maturity parameters.

The unconformity surface at the bottom of the Paleogene is one of the most important migration pathways in the Sikeshu Sag of the Junggar Basin, which consists of three layers: upper coarse clastic rock, lower weathering crust and leached zone. The upper coarse clastic rock is characterized by higher density and lower SDT and gamma-ray logging parameters, while the lower weathering crust displays opposite features. The transport coefficient of the unconformity surface is controlled by its position in respect to the basal sandstone; it is higher in the ramp region but lower in the adjacent uplifted and sag areas. The content of saturated hydrocarbons increases with the decrease of the content of non-hydrocarbons and asphaltenes. The content of benzo[c] carbazole decreases as the content of benzo[a] carbazole and [alkyl carbazole]/[alkyl + benzo carbazole] increases. This suggests that the unconformity surface is an efficient medium for the transportation of hydrocarbons.Graphical abstractDownload full-size imageHighlights► The Paleogene Sikeshu unconformity surface consists of upper clastic, lower weathered and leached beds. ► The transport coefficient of the unconformity surface is higher at ramp and lower in uplift/sag. ► The unconformity surface provides efficient hydrocarbon transportation and accumulation.

•Hydrocarbon potential of the Abu Gabra source rocks in the Sufyan Sag was evaluated.•Their hydrocarbon generation and expulsion content was estimated by basin modeling.•The AG3 and AG2 source rocks supplied most of the hydrocarbons in the Sufyan Sag.•The AG1 source rock has no effective hydrocarbon generation and expulsion process.•The Southern Sub-sag is the main source kitchen contributing 90% of the hydrocarbons.The Sufyan Sag is one of the low-exploration areas in the Muglad Basin (Sudan), and hydrocarbon potential evaluation of source rocks is the basis for its further exploration. The Abu Gabra Formation consisting of three members (AG3, AG2 and AG1 from bottom to top) was thought to be the main source rock formation, but detailed studies on its petroleum geology and geochemical characteristics are still insufficient. Through systematic analysis on distribution, organic matter abundance, organic matter type, organic matter maturity and characteristics of hydrocarbon generation and expulsion of the source rocks from the Abu Gabra Formation, the main source rock members were determined and the petroleum resource extent was estimated in the study area. The results show that dark mudstones are the thickest in the AG2 member while the thinnest in the AG1 member, and the thickness of the AG3 dark mudstone is not small either. The AG3 member have developed good-excellent source rock mainly with Type I kerogen. In the Southern Sub-sag, the AG3 source rock began to generate hydrocarbons in the middle period of Bentiu. In the early period of Darfur, it reached the hydrocarbon generation and expulsion peak. It is in late mature stage currently. The AG2 member developed good-excellent source rock mainly with Types II1 and I kerogen, and has lower organic matter abundance than the AG3 member. In the Southern Sub-sag, the AG2 source rock began to generate hydrocarbons in the late period of Bentiu. In the late period of Darfur, it reached the peak of hydrocarbon generation and its expulsion. It is in middle mature stage currently. The AG1 member developed fair-good source rock mainly with Types II and III kerogen. Throughout the geological evolution history, the AG1 source rock has no effective hydrocarbon generation or expulsion processes. Combined with basin modeling results, we have concluded that the AG3 and AG2 members are the main source rock layers and the Southern Sub-sag is the main source kitchen in the study area. The AG3 and AG2 source rocks have supplied 58.1% and 41.9% of the total hydrocarbon generation, respectively, and 54.9% and 45.1% of the total hydrocarbon expulsion, respectively. Their hydrocarbon expulsion efficiency ratios are 71.0% and 62.3%, respectively. The Southern Sub-sag has supplied more than 90% of the total amounts of hydrocarbon generation and its expulsion.

•The present heat flow map of Sufyan Depression is drawn based on 13 wells.•The heal flow history of Sufyan Depression is predicted.•The Ro (%) history of AG source rocks is simulated.•The Ro (%) maps of AG-2_down and AG-3_up source rocks are completed and the results match the present oil shows.The Sufyan Depression is located in the northwest of Muglad Basin and is considered as a favorable exploration area by both previous studies and present oil shows. In this study, 16 wells are used or referred, the burial history model was built with new seismic, logging and well data, and the thermal maturity (Ro, %) of proved AG source rocks was predicted based on heat flow calculation and EASY %Ro modeling. The results show that the present heat flow range is 36 mW/m2∼50 mW/m2 (average 39 mW/m2) in 13 wells and 15 mW/m2∼55 mW/m2 in the whole depression. Accordingly, the geothermal gradient is 20 °C/km∼26 °C/km and 12 °C/km∼30 °C/km, respectively. The paleo-heat flow has three peaks, namely AG-3 period, lower Bentiu period and Early Paleogene, with the value decreases from the first to the last, which is corresponding to the tectonic evolution history. Corresponding to the heat flow distribution feature, the AG source rocks become mature earlier and have higher present marurity in the south area. For AG-2_down and AG-3_up source rocks that are proved to be good-excellent, most of them are mature with Ro as 0.5%–1.1%. But they can only generate plentiful oil and gas to charge reservoirs in the middle and south areas where their Ro is within 0.7%–1.1%, which is consistent with the present oil shows. Besides, the oil shows from AG-2_down reservoir in the middle area of the Sufyan Depression are believed to be contributed by the underlying AG-3_up source rock or the source rocks in the south area.

•Three charging episodes (142–139 Ma, 128–122 Ma and 114–106 Ma) were identified.•Source rocks control the distribution of tight oil reservoirs.•Tight reservoir beds form the continuous “retained-accumulation”.•Different source-reservoir assemblages lead to different charging degrees.•PDBSR provided the main expulsion and charging force for crude oil.Tight sandstone oil reservoirs have received increasing attention in petroleum exploration and exploitation. Previous research concerning tight oil reservoirs has predominantly focused on marine basins. However, in China, tight oil reservoirs are mostly distributed in lacustrine basins. Taking the tight oil sandstones of the Yanchang Formation Chang 7 oil reservoir interval (the Chang 7 for short) in the Ordos Basin as an example, five factors are identified that control their formation and occurrence. Firstly, three episodes of oil charging took place during the Early Cretaceous, which were 142–139 Ma, 128–122 Ma and 114–106 Ma, respectively. Continuous generation and episodic expulsion led to multiple episodes of oil accumulation. Secondly, widely distributed source rocks (TOC > 1.0%) provide hydrocarbons to the Chang 7. The distribution of tight oil was controlled by the outer boundary of the source rock distribution, while the transition areas between generation (expulsion) centers were the accumulation and enrichment zones in the Chang 7. Thirdly, with the favorable conditions for retained hydrocarbons, massive continuous tight reservoir beds (average porosity = 7.6% and average permeability = 0.15 mD) developed in the Chang 7 capable of forming large-scale successive tight oil reservoirs. Fourthly, different source-reservoir assemblages have different expulsion patterns and amounts, which led to different charging degrees and sizes of tight oil accumulations. Type III (source-reservoir interbeds) will be the most favorable target for exploration and exploitation in the Chang 7, Ordos Basin. Finally, under the driving mechanism of non-buoyancy migration and accumulation, pressure differences between source and reservoir rocks (PDBSR) serve as the primary expulsion and migration force for crude oil during the main accumulation periods in the Chang 7.