Hydrocarbon Trap Research Articles

Linking structures with the genesis and activity of mud volcanoes: examples from Emilia and Marche (Northern Apennines, Italy)

Mud volcanism is known to be strictly linked to tectonic structures, since they have the ability to trap hydrocarbon and other fluids, and eventually induce them to migrate from a deep reservoir (e.g. sited into an anticline core, where large overpressures may be generated), to the surface. A central theme is how fluids can migrate upward, and which is the role in this process of near structures (i.e. structures directly controlling the mud volcano system) and far structures (i.e. faults located far away from the mud volcano system). In this study, we investigate the role of both types of structures in the genesis and evolution of mud volcanoes. In particular, we investigate six mud volcano case studies from the Emilia-Romagna and Marche Pede–Apennine margin, in Italy, through integration of fieldwork, interpretation of available seismic reflection profiles and aerial photos. The results of these analyses support an intimate link of the investigated mud volcanoes with anticline structures. We discuss two different fluid migration settings, particularly (i) mud volcanoes emplaced on outcropping anticlines, and (ii) mud volcanoes located on top of buried structures, discerning when fluids are likely to exploit anticline-related fracture sets, or secondary structures and porosity. Finally, we speculate on how far structures, via the seismic triggering, may play a role in the occurrence of historical eruption of some of the investigated mud volcanoes.

Purification of exhaust gases from gasoline engine using adsorption-catalytic systems. Part 1: trapping of hydrocarbons by Ag-modified ZSM-5

In the present work, adsorptive-catalytic properties and thermal stability of the systems based on ZSM-5 zeolite and γ-Al2O3, used as a binder, were studied. The zeolite was modified with silver, and the Ag loading used was 5 and 10 wt%. Characterization of the samples by a transmission electron microscopy has revealed that for the 5%Ag/ZSM-5 sample the predominant part of silver is represented by ion-exchanged forms. The 10%Ag/ZSM-5 sample contains also silver nanoparticles and clusters on the outer zeolite surface. As it was found by means of X-ray diffraction analysis, all these surface Ag species are roentgen-amorphous. Diffuse reflectance UV–vis spectroscopy confirms presence of the surface Ag clusters, which concentration increases after the high-temperature aging. The adsorption capacity of the samples was estimated using toluene as a model aromatic compound. The composition of the reaction mixture during the aging procedures was varied in order to elucidate the contribution of different silver sites to hydrocarbons sorption, their partial oxidation and oxidation of carbon monoxide.

Investigating the effect of [C8Py][Cl] and [C18Py][Cl] ionic liquids on the water/oil interfacial tension by considering Taguchi method

Capillary and interfacial forces are of great influences of trapping hydrocarbon in porous media after primary and secondary recovery processes. The trapped crude oil in the reservoir can be mobilized and produced by reducing these forces. Thus, surfactant flooding, as a main enhanced oil recovery (EOR) method, is usually applied to reduce the interfacial tension (IFT) of crude oil–water system in porous medium and improves the oil recovery. This study focused on the effect of [C8Py][Cl] and [C18Py][Cl] ionic liquids (ILs), as a new family of surfactant, in combination with various salts including sodium chloride, potassium chloride, magnesium sulfate and potassium sulfate on IFT reduction. EOR injection solutions were prepared from mixing the ILs at different concentrations of 100, 250, 500 and 1000 ppm with the salts ranging from 500 to 80,000 ppm. Obtained results showed that the minimum IFT value from both ILs was achieved when the concentration of the ILs was about 1000 g/mL, and the concentrations of KCl, K2SO4, MgSO4 and NaCl were 1000, 2000, 500 and 80,000 ppm, respectively. The minimum IFTs were achieved when NaCl and ILs concentrations were the maximum and MgSO4 concentration was the minimum.

CO2 Stripping of Kerogen Condensates in Source Rocks

SummarySignificant research has been conducted on hydrocarbon fluids in the organic materials of source rocks, such as kerogen and bitumen. However, these studies were limited in scope to simple fluids confined in nanopores, while ignoring the multicomponent effects. Recent studies using hydrocarbon mixtures revealed that compositional variation caused by selective adsorption and nanoconfinement significantly alters the phase equilibrium properties of fluids. One important consequence of this behavior is capillary condensation and the trapping of hydrocarbons in organic nanopores. Pressure depletion produces lighter components, which make up a small fraction of the in-situ fluid. Equilibrium molecular simulation of hydrocarbon mixtures was carried out to show the impact of CO2 injection on the hydrocarbon recovery from organic nanopores. CO2 molecules introduced into the nanopore led to an exchange of molecules and a shift in the phase equilibrium properties of the confined fluid. This exchange had a stripping effect and, in turn, enhanced the hydrocarbon recovery. The CO2 injection, however, was not as effective for heavy hydrocarbons as it was for light components in the mixture. The large molecules left behind after the CO2 injection made up the majority of the residual (trapped) hydrocarbon amount. High injection pressure led to a significant increase in recovery from the organic nanopores, but was not critical for the recovery of the bulk fluid in large pores. Diffusing CO2 into the nanopores and the consequential exchange of molecules were the primary drivers that promoted the recovery, whereas pressure depletion was not effective on the recovery. The results for N2 injection were also recorded for comparison.

2D seismic interpretation and hydrocarbon prospects for the Neogene-Quaternary succession in the Temsah Field, offshore Nile Delta Basin, Egypt

This study is concerned with the Neogene-Quaternary subsurface sedimentary succession of the Temsah Field in the northeastern offshore Nile Delta Basin. A sequence stratigraphic analysis based on seismic and available well data enables the subdivision of the investigated succession into two megasequences: the Pre- and Post-Messinian Megasequences. The Pre-Messinian Megasequence encompasses Miocene rock units, whereas the Post-Messinian Megasequence includes Pliocene-Pleistocene rock units. The detailed sequence stratigraphic analysis presented herein subdivides the Pre-Messinian Megasequence into three 3rd-order depositional sequences as follows (from the base to the top): Sequence-1 (Serravallian Sidi Salim Formation), Sequence-2 (Tortonian Wakar Formation), and Sequence-3 (Messinian Rosetta Formation). The Post-Messinian Megasequence is also subdivided into three additional 3rd-order seismic sequences: Sequence-4 (Lower-Middle Pliocene Kafr El-Sheikh Formation), Sequence-5 (Late Pliocene El-Wastani Formation) and Sequence-6 (Pleistocene Mit Ghamr Formation). This study suggests that the sea level underwent six transgressive-regressive depositional cycles during the evolution of the Neogene-Quaternary succession. The greatest relative sea level fall occurred during the Messinian, while the highest identified relative sea level rise occurred during the Lower-Middle Pliocene. The interpreted NW-SE anticline represents a distinctive four-way closure that may form a suitable structural hydrocarbon trap within the sandy intervals of the siliciclastic Sidi Salim and Wakar Formations in the Temsah Field. Moreover, the identified single and multiple buried channels within the Kafr El-Sheikh Formation are recommended for further exploration and possible drilling.

Analysis of the fire-extinguishing effect and the weakening of flame intensification of nonionic liquid water mist*

ABSTRACTThe determination of a water mist additive is important to weaken the flame enhanced by water mist, which affects the safety and applicability of the water mist fire-fighting technology to a certain extent. Based on the theory of physical chemistry, in this study, a water mist additive, that is, a nonionic liquid alkyl phenol epoxide (APE) condensate to weaken the flame intensification and systematically perform an experimental study on extinguishing the fire in oil pool with nonionic liquid water mist. The results show that the nonionic liquid water mist has better atomization parameters. The extinguishing time and the maximum temperature are all decreased with the increase of nonionic liquid mass fraction under the action of nonionic liquid water mist. The maximum CO volume fraction and the time to maintain a high CO volume fraction in fire flue gas decreases with the increasing nonionic liquid mass fraction. The minimum O2 volume fraction and the recovery rate from this minimum value to the normal value increases with increasing nonionic liquid mass fraction. The nonionic liquid water mist can weaken the flame intensification and extinguish the oil pool fire better than pure water mist. This is because the suspended air and water bubbles formed after the spraying of the nonionic liquid water mist can block heat radiation, cutting off the oxygen supply and isolating the contact of combustibles with oxygen. In addition, owing to high temperature, surfactant and water molecules linked by hydrogen bond ether group rupture, resulting in reduced HLB, thus enhancing the ability to trap hydrocarbon molecules and capture H• and OH• radicals in the chain reaction.

Cretaceous extensional systems of the Golfo San Jorge basin: Insights from the analysis of fault length and displacement data

Displacement profiles and maps were used in the subsurface of the Golfo San Jorge basin in the Manantiales Behr region to analyze factors affecting the geometric and kinematic evolution of an extensional system and its relationship with the migration and trapping of hydrocarbons. Two fault types were recognized: faults affecting both the basement and the sedimentary cover, and faults restricted to the sedimentary cover. The vertical growth of the faults within the sedimentary cover was constrained by vertical lithologic variations and by intersection with the free surface. Faults involving the basement are synsedimentary and show Dmax/L relationships and aspect ratios that allow their distinction from the shallower fault planes. The geometric coherence shown in the aggregated displacement profile and map suggests a synchronous kinematic evolution for the faults developed inside the sedimentary cover. Observed fault trace lengths and throw values distributions may indicate that those faults grew by the early linkage of isolated fault segments followed by the progressive building of displacement. Reactivation of pre-existent basement fabric had little influence on faulting across overlying sedimentary cover probably because the fine-grained beds of Pozo D-129 Formation acted as a structural decoupling level. The low degree of connection between the shallow and deep portions of the faults disregards the hypothesis of vertical migration from a local source pod. The mapping of fault segments with higher values of displacement and the recognition of transfer zones may help to locate and delineate footwall hydrocarbon traps. Comparison and integration with other studies show that Cretaceous fault systems in the eastern Golfo San Jorge basin have grown mainly by rapid length establishment and subsequent displacement accumulation. Also, most of the basement-involved structures do not connect significantly with overlying fault arrays.

FTG Gravity data for prospecting and exploration play models

Geophysical technologies are routinely deployed to investigate and enhance exploration play models evoked to explain an economic discovery?s geological setting. Such models are diverse for different commodities, but two such models adopted for hydrocarbon exploration lend themselves particularly suited to gravity surveying methods. Salt bodies and 4-way structural closures are unique in that their 3D shape and near homogenous density present themselves as ideal hydrocarbon trapping environments. An added complexity occurs where these bodies exist in both onshore and transition zone settings as deployment of conventional geophysical methods such as seismic becomes cost prohibitive. The arrival of Airborne Gravity surveying capabilities in recent years now makes exploring such exploration play models viable. Air-FTG® offers a high precision, high accuracy technology that determines changes to the gravity field as sourced by sub-surface geology. The technology is ideal for mapping density contrasts due to structural faulting and or the presence of geological bodies of near uniform density as exhibited by salt and 4-way structural closures. This paper will present data examples demonstrating the usefulness of FTG Gravity when investigating the prospectivity of exploration play models.

Reply to Comment by F. Kenig, L. Chou, and D. J. Wardrop on “Evaluation of the Tenax Trap in the Sample Analysis at Mars Instrument Suite on the Curiosity Rover as a Potential Hydrocarbon Source for Chlorinated Organics Detected in Gale Crater” by Miller et al., 2015

AbstractKenig et al. comment on our 2015 reporting of laboratory analog experiments aimed at testing the stability of the hydrocarbon trap material used in the Sample Analysis on Mars (SAM) instrument on board the Curiosity Rover operating in Gale Crater on Mars. They propose chemical structures for some decomposition products of the Tenax TA polymer when it is exposed at high temperatures to the Cl2 and O2 gases formed by the thermal decomposition of perchlorate. Further, Kenig et al. propose that these decomposition products accumulate and then react further in cooler downstream sections of the SAM analytical pipeline to produce the chlorobenzene that was detected in the Cumberland mudstone of Gale Crater. However, numerous experiments conducted in the laboratory show that Tenax TA decomposition products only appear after repeated exposure to much higher levels of Cl2 and O2 than those seen by the flight instrument. Moreover, the sequence of chlorobenzene detections during gas chromatography‐mass spectrometry experiments conducted on Mars cannot be explained by Tenax TA decomposition, nor can the detection of chlorobenzene in Evolved Gas Analysis experiments that involve pathways devoid of Tenax TA. Kenig et al. are incorrect in their assertion that Tenax TA decomposition products can account for the chlorobenzene detected on Mars by SAM.

Performance of electrospun nanofibrous membranes for trapping of BTX aromatic hydrocarbons and heavy metal ions: Mechanisms, isotherms and kinetics

Fly ash (FA) was considered as an adsorbent for adsorbing some harmful substances because of its unique characteristics such as high specific surface area, porosity, functional groups, unburned carbon content in the ash, etc. In this study, FA powder with polyacrylonitrile (PAN) matrix was electrospun into a multi-functional nanofibrous membrane for trapping of BTX aromatic hydrocarbons (benzene, toluene, and xylene) and heavy metal ions. These obtained FA/PAN composites possessed the satisfactory adsorption capacity for BTX aromatic hydrocarbons and heavy metal ions including Co(II), Pb(II) and Cr(VI). The performance of electrospun PAN-based FA nanofibrous membranes for trapping of BTX compounds and heavy metal ions was investigated with emphasis on the mechanisms, isotherms and kinetics. The impact of specific surface area, pore structure, porosity and functional groups on the adsorption behavior as well as adsorbate-adsorbent physicochemical interactions (e.g., electrostatic attraction, nonpolar attraction) were discussed. The results showed that all the tested materials had the strongest adsorption capacity for trapping of xylene, followed by toluene and benzene, and for trapping of heavy metal ions followed the order: Pb(II) > Co(II) > Cr(VI). The most exciting result is that although the trapping of BTX aromatic hydrocarbons by FA/PAN nanofibrous membrane was slightly lower than that by activated carbon (AC) powder, it was about 1.48 and 5.04 times higher than that by AC powder for trapping of Co(II) and Pb(II), respectively. In addition, the initial adsorption rate of FA/PAN membrane and FA powder for heavy metal ions was significantly faster than that of AC powder.

Integrated analysis of fault sealing capabilities when studying and mapping hydrocarbon traps in the Dnieper-Donets Depression

This paper demonstrates how a set of advanced technologies provided a better understanding of the geological structure of the Solokha gas condensate field and underground gas storage located in onshore Ukraine. In particular, these technologies helped us improve the process of hydrocarbon trap delineation and subsequent exploration well planning in this area. The study area is structurally complex because it is located in the central part of the axial zone of the Dnieper-Donets Depression (DDD). This part of the DDD is characterized by active halokinesis that has produced morphologically developed salt bodies and associated with them plicative and disjunctive deformations of the host rocks. The structure of the area has to a large extent been formed under the impact of one of such bodies, the closely located and actively developed Bakeyski diapir. The workflow used consists in synergic integration of high-quality 3D seismic data processing, detailed structural and stratigraphic interpretation and seismic inversion combined with pore pressure measurements in wells, lithotype clustering using Kohonen self-organizing maps, and fault seal analysis. A ramified network of faults, which plays an important role in the study area, formed independent tectonic traps in the Middle Jurassic sediments of the Bayosian age and led to the compartmentalization of one of the stratigraphic traps in the Lower Carboniferous sediments of the Visean age. To analyze sealing capabilities of these faults, Allan diagrams and Shale Gouge Ratio are used. The first of these methods is based on the juxtaposition of permeable and impermeable rocks on both sides of the fault, and the second, more advanced, is based on the calculation of the percentage of the clay component of all rocks that have slipped along the fault past a given point.

The impact of near-wellbore wettability on the production of gas and condensate: Insights from experiments and simulations

Condensate banking commonly occurs in gas-condensate reservoirs resulting in a reduction in well productivity. This phenomenon is linked to the phase behavior of the reservoir fluid, the drawdown management technique, as well as the trapping of liquid hydrocarbons due to strong rock-fluid interactions. Wettability alteration to neutral-wetting near the wellbore has been identified as a possible solution to alleviate the problem. In this work, we investigate optimizing wettability conditions towards maximizing productivity. A range of wetting conditions is examined, experimentally, and using numerical simulation techniques. The results show that it is possible to target an optimum wettability state that maximizes gas well deliverability. A signature of these optimum conditions is identified in the trend of spontaneous imbibition tests. Complete reversal of wetting conditions can result in better well productivity than the original wetting state, but in some cases it can reduce the amount of condensate recovered. This puts into question the common perception that an increase in the recovery of heavy components should be expected when liquid-wetting is reduced.

Editorial to the special Special Volume of Global and Planetary Change “Coupled Deep Earth and Surface Processes”

Lake-floor morphologies may be significantly different from seafloor topographies of other basins, typically observed in passive or active continental margins. The bathymetry of large paleo-lakes is often overwritten by subsequent tectonic evolution, burial beneath thick overburden and inherent compaction effects. We study the evolution of such an initial underfilled, balance fill and finally overfilled large paleo-lake basin by the interpretation of 2D and 3D seismic data set corroborated with calibrating wells in the example of the Neogene Pannonian Basin of Central Europe. Lake Pannon persisted for about 7–8 Myr and was progressively filled by clastic material sourced by the surrounding mountain chains and transported by large rivers, such as the paleo-Danube and paleo-Tisza. We combined sedimentological observations with a backstripping methodology facilitated by well lithology and porosity data to gradually remove the sediment overburden. This approach has resulted in a morphological reconstruction of the former depositional surfaces with special focus on the prograding shelf-margin slopes. Our calculations show that the water depth of the lake was more than 1000 m in the deepest sub-basins of the Great Hungarian Plain of the Pannonian Basin. The significant compaction associated with lateral variations of Neogene sediment thicknesses has created non-tectonic normal fault offsets and folds. These features have important effects on fluid migration and hydrocarbon trapping. We furthermore compare the geometries and effects of such non-tectonic features with the activity of larger offset sinistral strike-slip zones using 3D seismic attributes.

다양한 여재를 이용한 자동차 증발가스 저감용 Hydrocarbon-트랩에 대한 연구

다양한 여재를 이용한 자동차 증발가스 저감용 Hydrocarbon-트랩에 대한 연구

Impact of structural setting on hydrocarbon trapping mechanism of onshore Niger Delta basin, Nigeria, using seismic attribute analysis

The evaluation of hydrocarbon trapping system in an onshore Niger Delta basin, Nigeria was studied using well log and seismic data. This is very important in estimation of production cost and assessment of viable reservoir units in petroleum industry. It is the main aim in reservoir characterization, in order to estimate the geometry and trapping mechanism of any particular field under investigation. These were achieved through the integration of 3D seismic and well log data that was acquired from an Onshore Niger Delta basin, Nigeria. From the well logs, three reservoir units was identified namely RS1, RS2, RS3 and these were mapped based on the curve signatures of the geophysical logs. These identified hydrocarbon bearing intervals were mapped on the seismic section using checkshot dataset for time to depth conversion. Four horizons and a total of thirteen fault traces were mapped within the region, with two characterized building structural faults of F2 and F3 features. Fault F4 and F7 are antithetic fault that trends north east to south west and the trapping mechanisms for the region are both a four way closure and fault assisted closure, as the trap is an up dip fault closure against the up thrown side of fault F8. Amplitude, variance and coherence attributes shows the faulting structure of hydrocarbon migration channels and path ways, through which hydrocarbon migration and accumulation occurs within the region. A significant level of contribution was achieved with the variance attributes showing a better resolution of an instant fault detection or identification compared to other attributes studied within the field of interest. The identified hydrocarbon bearing units lies within the same depth across the entire wells. This proved that the region has exhibits little or no tectonic deformation. The presence of structural features such as antithetic and synthetic is responsible for the hydrocarbon entrapment within the region. A detailed analysis of the study location revealed highly faulted region that exhibits a typical Niger Delta tectonic settings.

Hydrodynamics and hydrocarbon trapping: Concepts, pitfalls and insights from case studies

Abstract Tilted contacts are observed in many oil and gas fields. They are generally attributed to the effect of hydrodynamics. Hydrodynamic flow exists in almost all geological basin types of the Wilson cycle, from passive margins to foreland basins to mountain chains. There are various geological processes driving regional hydrodynamics. Flow is regulated by permeability variations which are given by the geological framework. The role of faults as partial permeability barriers may be a significant regulator for flow. Hydrodynamics is best recognized by using the principles of hydraulic head. However, hydraulic data must be carefully interpreted to avoid pitfalls such as transition zones where variable oil saturation may be falsely interpreted as tilted contacts. In sedimentary basins, we distinguish two different hydrodynamic settings where flow is either mainly gravity-driven (or centripetal), or mainly compaction-driven (or centrifugal). In both cases, hydrodynamics can enhance hydrocarbon trapping. Gravity driven processes are particularly important in the Arabian Shield and North Africa. Centrifugal and centripetal phenomena can co-exist in the same basin, e.g. Mahakam delta or Douala Basin. Turbidites occur typically in centrifugal settings. Hydrodynamic concepts applied to petroleum exploration are not frequently used in industry despite being known for a long time. They may however become increasingly important in conventional exploration as static plays are rapidly being drilled up and the remaining exploration potential of sedimentary basins is increasingly affected by stratigraphic and hydrodynamic trapping components.

A numerical approach for investigation of stress states induced by salt structures

The main oil and gas reserves around the world are found in regions containing evaporitic rocks. Owing to their low porosity and low permeability these rocks provide favorable conditions for hydrocarbon trapping, increasing the probability of success in oil and gas exploration. Salt rocks exhibit time-dependent strain behavior when submitted to deviatoric stresses. This behavior is described by creep constitutive laws based on deformation mechanics. One of the major challenges in modeling stress states in sedimentary basins in the presence of salt structures is to predict the magnitude of stress perturbations around salt bodies. This work presents a geo-mechanical approach to take into account the specific weight variation throughout the lithology and the effects of buoyancy in the resulting forces on the interface salt-body/formation. Modeling is driven by the fact that salt bodies cannot sustain deviatoric stresses. Numerical analyses using the Finite Element Method under plane strain conditions provide the results for the comparison of the methodologies found in the literature with the one proposed by the authors. Two synthetic models with salt dome structures are investigated. The proposed approach provided more realistic stress fields in the vicinity of salt structures.

Passive Hydrocarbon Trap to Enable SULEV-30 Tailpipe Emissions from a Flex-Fuel Vehicle on E85 Fuel

Passive Hydrocarbon Trap to Enable SULEV-30 Tailpipe Emissions from a Flex-Fuel Vehicle on E85 Fuel

Strike-Slip Fault Deformation and Its Control in Hydrocarbon Trapping in Ketaling Area, Jambi Subbasin, Indonesia

Geologically, Ketaling area consists of a local high considered as flexure margin of Tempino-Kenali Asam Deep in west part and graben in east part also known as East Ketaling Deep. Numerous proven plays were established in Ketaling area with reservoir in early Miocene carbonate and middle Miocene sand. This area underwent several major deformations. Faults are developed widely, yet their geometrical features and mechanisms of formation remained so far indistinct, which limited exploration activities. With new three-dimensional seismic data acquired in 2014, this area evidently interpreted as having strike-slip mechanism. The objective of this study is to examine characteristic of strike slip fault and its affect to hydrocarbon trapping in Ketaling Area. Structural pattern and characteristic of strike slip fault deformation was examined with integration of normal seismic with variance seismic attribute analysis and the mapping of Syn-rift to Post-rift horizon. Seismic flattening on 2D seismic cross section with NW-SE direction is done to see the structural pattern related to horst (paleohigh) and graben. Typical flower structure, branching strike-slip fault system and normal fault in synrift sediment clearly showed in section. An echelon pattern identified from map view as the result of strike slip mechanism. Detail structural geology analysis show the normal fault development which has main border fault in the southern of Ketaling area dipping to the Southeast-East with NE-SW lineament. These faults related to rift system in Ketaling area. NW-SE folds with reactive NE-SW fault which act as hydrocarbon trapping in the shallow zone. This polyphase tectonic formed local graben, horst and inverted structure developed a good kitchen area (graben) and traps (horst, inverted structure). Subsequently, hydrocarbon accumulation potentials such as basement fractures, inverted syn-rift deposit and shallow zone are very interesting to explore in this area.

Fluid geochemistry of the Jurassic Ahe Formation and implications for reservoir formation in the Dibei area, Tarim Basin, northwest China

The condensate gas reservoirs of the Jurassic Ahe Formation in the Dibei area of the Tarim Basin, northwest China are typical tight sandstone gas reservoirs and contain abundant resources. However, the hydrocarbon sources and reservoir accumulation mechanism remain debated. Here the distribution and geochemistry of fluids in the Ahe gas reservoirs are used to investigate the formation of the hydrocarbon reservoirs, including the history of hydrocarbon generation, trap development, and reservoir evolution. Carbon isotopic analyses show that the oil and natural gas of the Ahe Formation originated from different sources. The natural gas was derived from Jurassic coal measure source rocks, whereas the oil has mixed sources of Lower Triassic lacustrine source rocks and minor amounts of coal-derived oil from Jurassic coal measure source rocks. The geochemistry of light hydrocarbon components and n-alkanes shows that the early accumulated oil was later altered by infilling gas due to gas washing. Consequently, n-alkanes in the oil are scarce, whereas naphthenic and aromatic hydrocarbons with the same carbon numbers are relatively abundant. The fluids in the Ahe Formation gas reservoirs have an unusual distribution, where oil is distributed above gas and water is locally produced from the middle of some gas reservoirs. The geochemical characteristics of the fluids show that this anomalous distribution was closely related to the dynamic accumulation of oil and gas. The period of reservoir densification occurred between the two stages of oil and gas accumulation, which led to the early accumulated oil and part of the residual formation water being trapped in the tight reservoir. After later gas filling into the reservoir, the fluids could not undergo gravity differentiation, which accounts for the anomalous distribution of fluids in the Ahe Formation.