Organic-rich Intervals Research Articles

Study on the relationship between shale organic matter development and paleowater depth—A new understanding of the condensed section

Recent studies dispute the long-standing belief that highly organic-rich shales are predominantly linked to condensed section (CS) at the maximum flooding surface (MFS), proposing instead that they more commonly develop in shallow waters along basin margins. This hypothesis, however, has not been thoroughly investigated. Our study, focusing on the pivotal regions of the Longmaxi Formation and the Chang 7 Member for marine and continental shale exploration in China, integrates sequence stratigraphy with analyses of total organic carbon (TOC) and elemental geochemistry to pinpoint the environments conducive to the formation of organic-rich shales. The most organic-rich intervals, specifically the bottom of the Longmaxi Formation and the mid-to-lower Chang 73 sub-member, are located within the transgressive systems tract (TST), showing higher TOC levels in the middle to lower TST, compared to the CS of the MFS. The formation of high organic matter shale is related to warm, humid, and dysoxic shallow waters with minimal terrigenous influx and high primary productivity. We found a pronounced inverse relationship between organic matter concentration and water depth, with deeper waters promoting the decomposition and recycling of organic matter during sedimentation, thereby diminishing the TOC content. Moreover, volcanic activity also affected the organic enrichment in the Chang 7 Member. With high primary productivity, organic matter tends to accumulate in dysoxic slope zones. Consequently, high-organic content shale intervals are generally situated in shallow, dysoxic water environments, suggesting that the CS of the MFS may actually represent organic-poor intervals with limited source rock potential. The results of this study are of great significance for shale oil and gas exploration in China, and provide a new direction for the discovery of shale oil and gas sweet spots.

An integrated stratigraphic re-evaluation of key Central Atlantic DSDP sites

Re-sampling and new analysis of DSDP core from key Central Atlantic sites, drilled over 30 years ago, has allowed an updated stratigraphic framework to be developed. This study refines the timing and nature of key unconformities that have previously been documented to occur elsewhere in the oceanic domain of the Central Atlantic during the drilling of many DSDP legs. Biostratigraphic dating using calcareous nannofossils and palynology has been performed on 168 samples. This higher biostratigraphic resolution improves regional correlation and has allowed identification of major hiati previously unrecognised within the oceanic domain stratigraphy.This work builds on the pioneering work of the DSDP cruises, and utilises the valuable data collected for new age dating, supplemented with new organic geochemistry analysis and sedimentological characterisation to improve documentation of the Mesozoic strata. Results are extrapolated along the continental margin of northwest Africa and the conjugate in South America, using regional two-dimensional seismic sections, providing a correlation between the oceanic domain stratigraphy and shelfal depositional systems penetrated by hydrocarbon exploration wells.At DSDP Site 367, a middle Berriasian unconformity is recognised for the first time in Core 33, where middle Berriasian white pelagic limestones (Blake-Bahama Formation) sit unconformably on Early Tithonian red argillaceous limestones (Cat Gap Formation). Faunal dating indicates a ca. 5 Myr hiatus at the Jurassic-Cretaceous boundary. Marl interbeds within the pelagic limestones increase in occurrence through the Barremian, reflecting increased terrigenous input, with associated total organic content (TOC) averaging 5%.A second major lithological break is identified at DSDP Site 367, interpreted as a base Albian unconformity, between the Blake-Bahama and overlying Hatteras Formations. Sediments above are intra late Albian variegated claystones with abundant terrestrial detritus and distal turbidites. Seismic data that intersects the well location show onlapping reflections of early-middle Albian strata landward of DSDP Site 367, indicating paleo-topography prior to Albian deposition. These sediments are contemporaneous with a major progradational phase on the shelf and a sedimentary wedge observed at the base of the carbonate escarpment. Early Albian sediments sit above the interpreted base Albian unconformity at DSDP Site 534 A on the conjugate US Atlantic margin, suggesting the super-regional nature of this event. The refined dating provides age calibration of a basin-wide unconformity, previously known from seismo-stratigraphic studies as Reflector β.In addition to the revised stratigraphic framework, evidence is presented for the occurrence of seaward dipping reflectors (SDRs) northwest of the Guinea Plateau. SDRs have not been reported along this section of the African margin, demonstrating volcanic addition during early seafloor spreading and extending the occurrence of volcanics associated with the African Blake Spur magnetic anomaly further south beyond the Cap Vert fracture zone previously mapped. At DSDP Site 368 we also note that the late Albian to late Cenomanian strata (Hatteras Formation) is composed of a 164 m thick organic-rich interval, containing up to 37% TOC, with facies and organic geochemical heterogeneity. This transgressive sequence is associated with a flooding of the shelf.This revised stratigraphic framework provides a solid foundation for future Central Atlantic studies intending to use these sites as calibration of the oceanic domain stratigraphy.

Paleoenvironmental reconstruction of the Berriasian organic-rich interval of the Vaca Muerta Formation (Neuquén Basin, Argentina): Insights for the characterization of unconventional hydrocarbon shale reservoirs

The 50 m-thick Berriasian organic-rich interval (BORI) of the Vaca Muerta Formation (Neuquén Basin, Argentina) represents one of the most attractive stratigraphic intervals for unconventional hydrocarbon exploration in the unit. Nevertheless, little is known about the environmental conditions prevailing during the accumulation of the sediments. To unravel the causes that favored the preservation of the organic matter (OM) in the BORI, a detailed multiproxy sedimentological approach was conducted. The study included the analyses of microfacies, palynological content, calpionellid-calcisphere dinocysts, mineralogy, spectral gamma-ray, inorganic geochemistry, Rock-Eval pyrolysis and stable carbon isotopes of whole rock carbonate (δ13Ccarb), and associated OM (δ13Corg). The BORI presents a late Early to early Late Berriasian age and is subdivided into a lower and upper interval. The lower interval (25.6 m-thick) is primarily formed by radiolaritic wackestones and presents higher total organic carbon (TOC) content (4.2 wt% on average), mainly in the form of amorphous OM (AOM), with variable contribution of phytoclasts. The geochemical analyses (Si, Ni, Cu, Mo, U, V) indicate high productivity of the water column and overall sea bottom anoxia. Oppositely, the upper interval (24.4 m-thick) is mostly constituted by peloidal packstones/grainstones and presents a lower TOC content (1.5 wt% on average) mainly constituted by phytoclasts, with variable contribution of AOM. A decrease of the productivity and an increase of sea bottom oxygenation is recorded based on the geochemical analyses. The high OM content of the BORI responds to the combination of high productivity of the water column and sea bottom anoxia in response to the Berriasian transgression and a worldwide paleoclimatic change towards more humid conditions. The results of our study position the BORI as an attractive interval for unconventional hydrocarbon production due to its high TOC, low bulk clay mineral and high biogenic quartz content, granting an adequate geomechanical behavior.

Origin analysis of paleo-overpressure in source rocks based on logging response characteristics: a case study of the Chang 7 member in the Triassic Yanchang formation, Longdong area, Ordos Basin, China

The identification of the origins of source rock palaeo-overpressure based on logging response parameters is frequently influenced by the presence of organic matter. To accurately identify the paleo-overpressure origin of source rocks and employ an appropriate method for paleo-pressure calculation, this study proposes a quantitative approach to assess the influence of organic matter content on overpressure analysis. The findings demonstrate that the presence of organic matter in Chang 7 shale leads to an increase in acoustic time difference and a decrease in density. After incorporating the multi-component rock equivalent volume model correction, it is evident that the paleo-overpressure observed in the Chang 7 member is not attributed to undercompaction but rather to fluid expansion mechanisms. In conjunction with the acoustic velocity-density palte method, the paleo-overpressure of the Chang 7 member is attributed to undercompaction and expansion resulting from hydrocarbon generation. The pressure evolution of the Chang 7 member has been reconstructed based on the identification of its paleo-overpressure origin. The distribution characteristics of paleo-pressure recovered based on hydrocarbon generation expansion exhibit a strong correlation with the reservoir. The reliability of the correction method for organic matter influence is demonstrated. The proposed method is not only applicable for the investigation of paleo-pressure, but also holds significant implications for the assessment of pressure within organic-rich intervals.

An Integrated Frac Geometry Control Approach To Prevent H2S Production

_ Hydrogen sulfide (H2S) production has in recent years become a mounting issue facing operators and midstream companies in major US basins. At present, H2S production appears to be random and poorly understood. As such, H2S is considered by many to be an unpreventable nuisance and one that is largely addressed by treating the dangerous gas at surface before putting it to a sales line. This article highlights impacts of H2S, possible sources, and a successful case study of a completions operation designed specifically to prevent H2S production in North Dakota’s Williston Basin. H2S and Its Impacts H2S is a colorless gas known for its pungent “rotten egg” odor at low concentrations. It is extremely flammable, highly toxic, poisonous, and corrosive. H2S is commonly encountered during the drilling and production of crude oil and natural gas, as well as in wastewater treatment, utility facilities, and sewers. H2S is well-known for its significant health and safety risks, along with substantial financial implications in the event of an accidental release. The substance ranks as the second leading cause of workplace gas inhalation deaths in the US. This is according to the US Bureau of Labor Statistics, which concluded that H2S caused 46 worker deaths between 2011 and 2017. Due to its corrosiveness, wells that produce even small volumes of H2S may compromise wellbore integrity and lead to premature failure of production equipment. Sour oil and gas is considered to have about 10% lower profit margin than traditional sweet Bakken crude (SPE 141434). An electrical submersible pump replacement can cost an operator up to $250,000, plus production downtime. Additionally, according to one leading chemical company, it costs $2,250 per day to treat 200 ppm H2S at a typical gas production rate of 2,000 Mcf/D. H2S Sources A recent study, URTeC 3852794, published by the Energy and Environmental Research Center (EERC) at the University of North Dakota suggests three possible causes of H2S: geo-mechanical, thermochemical, and biogenic. Geo-mechanical: Mission Canyon organic-rich intervals could be a source of H2S for a limited number of the Bakken producing wells via communication between reservoirs caused by out-of-zone stimulation work. This conclusion was evidenced by matching the δ34S isotope between 30 H2S producing wells and formation cores. Thermochemical sulfate reduction (TSR): Another possible mechanism of H2S generation involves the presence of anhydrites and hydrocarbons at a higher reservoir temperature of 110° to 150°C. This mechanism is plausible for wells with the δ34S in the range from +10 to +19‰. The EERC research team is currently investigating the TSR existence in the Williston Basin. Biogenic: This third mechanism of reservoir souring is questionable in the Bakken because of the high reservoir temperatures (110° to 150°C) and elevated salinity of the formation water (TDS 200,000 mg/L) making microbial growth unlikely, especially in the context of the predominance of heavier 34S isotope (δ34S of +2 to +19‰) in produced H2S.

Hydrocarbon Geological Characteristics and Factors Controlling Hydrocarbon Accumulation of Jurassic Da’anzhai Continental Shale

Continental shale in China is a key exploration target with regard to unconventional hydrocarbons. Systematic research on the mineral composition, organic geochemistry, and hydrocarbon mobility has been performed in the Da’anzhai (J1dn) lacustrine shale oil reservoirs, clarifying the factors controlling shale oil accumulation. The results suggest that J1dn consists of three sub-members, with an organic-rich interval developed within the second sub-member. Three types of lithological associations have developed within the organic-rich interval: Type 1 represents the interbedding relationship between shale and shell limestone and Type 2 represents shale with intercalated limestone, followed by Type 3. The brittleness index of the J1dn continental organic-rich shale is relatively low. The porosity of the Da’anzhai lacustrine shale ranges between 0.5% and 10.5% (average value of 5.89%). The porosity is predominantly due to inorganic pores, with a small amount being attributable to nanoscale microfractures and organic pores. The average porosity of the shell limestone is only 1.2%, but fractures at the micron and centimeter scales are well developed. The organic matter in the J1dn continental shale is mainly II1–II2, with maturity in the oil generation stage. The average oil saturation of the J1dn continental shale is 3.15%, with most samples having oil saturation of less than 4%. The J1dn continental shale has great exploration potential with regard to shale oil. Type 1 shale oil is affected by multiscale fractures, including bedding fractures, and has the best mobility. The high hydrocarbon generation capacity of lacustrine shale, coupled with the multiscale fractures within shell limestone and shale, is the principal controlling factor for hydrocarbon enrichment. Based on exploration practices, the Type 1 shale association may represent the optimal interval for future shale oil exploration in the Da’anzhai Member.

Nitrogen isotopic compositions of organic-rich shales (∼560 Ma) in the Chengkou region, South China: Implications for a stable and relatively large nitrate reservoir of the late Ediacaran ocean

The late Ediacaran period is a critical transitional period in Earth's history, marked by the evolution of the Ediacara biota (∼574 to 538.8 Ma). The marine redox state and nutrient level, having a close relationship with bioevolution, can be reflected by sedimentary nitrogen isotopes (δ15N). Doushantuo Formation Member IV, a widespread organic-rich interval during this critical period of South China, may provide important clues for nitrogen (N) cycling. However, high-resolution δ15N research on Member IV is still lacking. In this study, a drill core from the Chengkou sub-basin, with the well-developed Doushantuo Member IV, was carefully studied base on its N isotopic composition. The implications for the N cycling characteristics are discussed using a combination of proxies for bottom water redox conditions and basin openness. The total nitrogen (TN) concentrations (up to 0.79 wt%) of these organic-rich samples are substantially higher than those of other counterpart profiles. The TOC content shows a positive linear relationship with TN concentration, and a significant positive TN-intercept (∼0.21 wt%) represents the addition of inorganic N produced by the remineralization of sinking organic N. The high TN concentrations may also be due to the flourishing and enhanced burial of primary producers. The δ15N value in the Doushantuo Member IV progressively decreases from ∼8.2‰ to ∼3.2‰ (from bottom to top), a result of the changes in basin openness and the enhancement of N2-fixation. Compilation shows the δ15N and δ13Corg values are approximately covariant, and both have significant temporal and spatial variations at this member of South China, indicating a strong heterogeneity in the water column redox environments and changes in the type of primary producers. A mode δ15N value of 4‰ indicates a stable and relatively large nitrate reservoir in the late Ediacaran ocean.

Influence of high-frequency lake level fluctuations on organic matter accumulation in the northern Qaidam Basin, NW China: Insights from spectral attribute analysis and geochemistry

Assemblages of multiple organic matter-rich sedimentary energy carriers are frequently found in Jurassic nonmarine basins of China. These basins are strongly influenced by high-frequency fluctuations in lake levels. A series of coal- and oil shale-bearing successions reflecting periodic lake level variations is also present in the Shimengou Formation, northern Qaidam Basin, NW China. These Middle Jurassic coal and oil shale seams provide an excellent basis for studying the effects of high-frequency lake level fluctuations on organic matter accumulation. Moreover, additional factors influencing the primary organic matter input (e.g., wildfires) and its preservation and later diagenetic transformation (e.g., processes during coalification) must be considered. In the present study, organic matter maturity, source potential, and paleoenvironmental conditions during source rock formation are investigated based on macro- and micropetrographic data, proximate analyses, bulk geochemical parameters, biomarker analyses, stable isotope geochemistry, and spectral attribute analysis to provide a sequence stratigraphic framework for the accumulation of organic-rich intervals. According to the established depositional model, the Shimengou Formation can be subdivided into four depositional sequences, including three organic matter-rich intervals. These include the thick mixed gas- and oil-prone Sq1 coal seam with a relatively high ash yield, which represents a highstand systems tract, the gas-prone Sq3 coal seam corresponding to a transgressive systems tract, and the Sq4 oil shale layers likely corresponding to a subsequent highstand systems tract, indicating a drowned mire. The results provide insights for the optimization of high-quality sedimentary mineral resource production in similar basins worldwide.

Environmental controls on organic matter enrichment of the lower Cambrian source rocks in the Tarim Basin, Northwest China

In the Tarim Basin, the largest petroliferous basin in China, a set of organic-rich deposition hosted in the lower Cambrian Yuertusi Formation is considered to be the main hydrocarbon source rocks. However, the limited and scattered distribution of the lower Cambrian constrains present studies mostly on some certain sections where samples are accessible, inhibiting a systematic understanding of characteristics of these source rocks and controls on organic matter enrichment. Here, we conduct a systematic study on a drillcore in northwestern margin of the Tarim Basin (Taxibei), integrated with previously published data from other available sections. These data show the Yuertusi Formation was mainly composed of mudstone/shale, carbonate and siliceous rocks, and distributed dominantly in the Taxibei, northern (Tabei) and eastern-northeastern areas of the Tarim Basin (Tadong-Tadongbei) with an increased thickness (mean 30 m to mean 60 m) towards the Tadong-Tadongbei areas. High total organic carbon (TOC) contents (up to 30%) were found in the mudstone/shale dominated intervals in both the Taxibei and Tabei areas, characterized by I-II kerogen and highly to over-matured thermal maturity. Paleo-redox reconstruction based on high V/(V + Ni) ratios (>0.6) and FeHR/FeT ratios (>0.38) suggests anoxic watermass conditions occurred during the deposition of the organic-rich intervals along with moderate sedimentation rate, providing consistently good preservation for organic matter accumulation. Based on a combined analysis, we infer primary productivity in surface waters exerted a key control on TOC contents of the organic-rich intervals, which was driven by nutrient supplies from hydrothermal activities with partial upwelling effects in the Taxibei area, and from upwelling waters in the Tabei area.

Barremian to Aptian Os isotopic record of a pelagic deep-sea chert (Goshikinohama-bedded chert) in a Japanese accretionary complex

The early Aptian oceanic anoxic event (OAE) 1a was one of the largest OAEs in the Cretaceous, and the organic-rich sediments have been reported from various sedimentation depths worldwide. Because the sedimentation age of this organic-rich interval corresponds to the radiometric ages of the Ontong Java Nui (OJN) basalts, their causal linkage has been debated. The Goshikinohama-bedded chert is a Cretaceous sedimentary sequence deposited at depths deeper than the carbonate compensation depth (∼4000 m) in the pelagic Pacific. Although it covers the Barremian to Aptian, its stratigraphic relation to the OAE1a has not been fully constrained. Here, we present the 187Os/188Os record of the Goshikinohama-bedded chert to further constrain the OAE1a interval. As a result, 187Os/188Os showed several shifts to unradiogenic values, which may be correlated to the early Aptian OAEs. However, the 187Os/188Os values of the Goshikinohama-bedded chert are less radiogenic than the previously reported Barremian to Aptian 187Os/188Os values. The less radiogenic trend may represent the high contribution of extraterrestrial Os in the sediments or the influence of hydrothermal fluid from the underlying Valanginian basaltic rocks. Because of such unknown factors, there remain uncertainties in the stratigraphic correlations between the Goshikinohama-bedded chert with other sections.

Outsized Turbidity Currents as a Primary Mechanism for Neoproterozoic Organic Carbon Delivery to the Deep Sea

Levees in modern deep-marine systems have been shown to sequester significant amounts of organic carbon due largely to their expanse and high rates of sedimentation. However, relatively few studies have examined organic carbon sequestration in ancient deep-marine leveed slope channel systems. Physical and geochemical analyses of well-exposed levee deposits in the Neoproterozoic Windermere Supergroup in B.C., Canada have shown that intervals of organic-rich (up to 4% TOC) strata correlate with conditions of elevated sea level and primary productivity on the shelf. Organic matter (OM) occurs primarily as micro- to nano-scale carbon adsorbed onto the surface of clay grains and notably occurs mostly in anomalously thick, mud-rich sandstone beds that are interspersed within successions of thin-bedded, comparatively organic-poor turbidites. The concentration of organic carbon in thick beds suggests that even when primary productivity is high it only becomes mobilized in significant quantities into the deep sea by uncommon, outsized turbidity currents. Although markedly more common in organic-rich intervals, thick, organic-rich beds occur also in organic-poor levee deposits, suggesting that the occurrence and frequency of outsized flows may be linked to primary productivity on the shelf. High rates of OM production and fallout would result in rapid accumulation of OM on the seafloor that then binds and provides mechanical strength to the accumulating sediment. Later this overthickened, organic-rich sediment pile becomes gravitationally unstable and ultimately remobilized downslope. These failure events create large, surge-like flows that are considerably thicker than the depth of the slope channels through which they travel. Accordingly, continuous overspill over the channel margins results in the deposition of an anomalously thick, sand- and organic-rich bed. These episodic events not only deplete the outer continental shelf of OM, but apparently also reduces the gradient slope of the local seabed, which then results in the more typical smaller, channel-confined organic-poor turbidity currents. Additionally, the abrupt and single-bed occurrence of OM-rich strata suggests that the buildup of organic-rich strata and seafloor stabilization was rapid but only of limited duration. Significantly, this study suggests that outsized turbidity currents that originate on the outer continental shelf are the primary mechanism for organic matter delivery to the deep sea, at least in pre-vegetation times, and that flow size and frequency, in addition to primary productivity, exerts an important control on the distribution of organic carbon in deep-sea sediments.

Effects of clay minerals and organic matter on pore evolution of the early mature lacustrine shale in the Ordos Basin, China

Pores related to organic matter and clay minerals provide significant storage space for hydrocarbons in organic-rich shales. However, the pore structure evolution and its related influencing factors at the early oil window stage are unclear. This study attempts to reveal the controls of organic matter and clay minerals on the pore structure of the lacustrine organic-rich Chang 7 Shale by using thin-section observational, organic petrological, elemental, scanning electron microscopy/Energy Dispersive Spectrometer (SEM/EDS) analyses, and mineralogical analyses combined with the pore structure characterization of gas adsorption and mercury injection capillary pressure (MICP). The results show that micropores (<2nm) and mesopores (2-50 nm) contribute to the main storage space in organic-rich intervals of the Chang 7 Shale. Organic matter content controls the development of micro-pores while no effects on porosity in the early mature lacustrine shale. Shrinkage OM pores related to the expulsion of hydrocarbons are better developed than the complex pores and sponge pores. Total clay content is correlated with mesopores volume in organic-rich lacustrine shale. Multiple stepwise linear regression model between mesopores volume and different clay minerals shows that the pores with diameters in the ranges of 2-5 nm are controlled by illite, chlorite, and kaolinite, and the pores with diameters in the ranges of 5-10 nm and 10-20 nm are controlled by illite, while 20-50 nm pores rely on illite/smectite mixed layer (I/S) and illite. Thermal maturity shows a positive effect on pore volume and porosity. There are three types of transformations of clay minerals in lacustrine shale, including the smectite-illite conversion, smectite-illite-chlorite conversion, and anorthite/albite-to-kaolinite/illite conversion. These clay mineral conversions all show positive effects on pore volume, especially the anorthite/albite-to-kaolinite/illite conversion, while the smectite-illite conversion shows some negative effects on the connectivity of the pore system. Additionally, the control of macropore volume, porosity, and pore throat radius by chlorite cannot be ignored. The findings from this study contribute to a clear insight into the pore structure evolution of lacustrine shale.

Pore system of the multiple lithofacies reservoirs in unconventional lacustrine shale oil formation

Unconventional lacustrine shale formations are characterized by strong heterogeneity and intensive lithological variations in the vertical direction. Nevertheless, most studies focused merely on the organic-rich intervals due to the successful exploration and development of marine shale gas resources. This study systematically analyzed the overall pore system of the Triassic Chang 7 lacustrine shale formation in the Ordos Basin, China, using geochemical, petrological, mineralogical and petrophysical methods. In addition, the statistical analysis method of spearman rank correlation was employed for the intensive data analysis. The results showed that rock components and diagenetic processes have significantly different effects on the development of multi-lithologic pore systems in lacustrine shale formations. Organic-rich laminated shale (ORLS) and argillaceous siltstone (AS) show a bimodal, while the medium organic matter massive shale (MOMMS), fine sandstone A (FSA), and coarse sandstone B (CSB) exhibit unimodal pore size distribution. Organic matter negatively affects the pore volume and positively influences the pore throat, while clay minerals generally play a negative role except for ORLS. Quartz is conducive to the pore development of the reservoir except that it blocks some pore space in ORLS. Meanwhile, feldspar boosts the pore volume of all reservoirs and inhibits the pore throat of ORLS and MOMMS. Calcite and dolomite have negative effects on the entire pore system. Pyrite and siderite could facilitate pore throat preservation but inhibit pore space development. The diagenetic analysis shows that thermal maturation of organic matter and mineral dissolution play important roles in promoting pore development, while cementation and mechanical compaction have counteracting effects. The assemblage of ORLS and CSB acts as the most favorable reservoirs for lacustrine shale oil exploration under ideal conditions. The pore size distribution of the lacustrine shale oil reservoir with random multi-lithologies shows that macropores and mesopores are the main reservoir space. This study contributes to a better understanding of sweet spot prediction in the notoriously complicated lacustrine shale formation.

Total organic carbon variability of lower Jurassic successions in the Mandawa Basin, SE Tanzania

Over many decades, an understanding of total organic carbon (TOC) variability in various sedimentary basins has been constrained by the availability of suitable samples for geochemical analysis. Many attempts rely on selected sidewall core or cuttings samples, which do not provide a continuous lithological record of the succession. As a result, TOC variability as a response to facies evolution is often underestimated. An advancement in TOC evaluation from well logging techniques often has permitted a continuous analysis of source rock intervals, while eliminating issues associated with obtaining discrete physical samples. Geochemical and wireline geophysical logs investigation here aimed at constraining TOC variability and depositional conditions prevailing during the early Jurassic in the Tanzanian Mandawa Basin (Mbuo, Nondwa and Mihambia Formations), which to date are poorly documented. TOC was deduced from both well cuttings samples and wire log data. Depositional conditions were defined via the use of spectral Gamma-Ray (Th/U, U) logging and lithofacies data. Sediment composition and TOC from cuttings were compared to illustrate how organic matter distribution is influenced by changes in sediment type. Complex mineral composition of heterolithic facies dominated log-response, complicated log interpretation, and thus limited the effectiveness of well logging techniques to predict TOC. Cuttings data reveal three organic-rich intervals, i) at the transition of Upper Mbuo to basal Nondwa Formation with the highest TOC of 3.5 (wt%), ii) in the Nondwa Formation with a TOC of 2 (wt%), and iii) in the Mihambia Formation with a TOC of 2.8 (wt%). An inverse linear relationship between Th/U and TOC signifies relatively shallow deposition in a well-mixed water column under oxic conditions. Intervals with higher TOC imply organic matter preservation under suboxic conditions upon two marine incursions in the Mihambia and Nondwa Formations, whereas a limnic environment prevailed at the Upper Mbuo to Nondwa transition. Inverse trends in TOC versus clastic components, a positive linear relationship between TOC and carbonates, and higher amount of clastics (>57 wt%) in the Mbuo and Mihambia Formations suggest a dominantly clastic sedimentary system with relatively stable carbonate and organic matter input in proximity to the source area. We, therefore, argue that the organic rich intervals observed in the Upper Mbuo, Nondwa and Mihambia Formations were deposited during periods of higher organic productivity driven by a rise in eustatic sea level, and coincide with a decline in terrigenous input from the hinterland.

Climate-controlled organic matter accumulation as recorded in the Upper Jurassic Argiles de Châtillon Formation, a shallow-marine counterpart of the Kimmeridge Clay Formation

AbstractMudstones from the Argiles de Châtillon Formation exposed in the Boulonnais region of Northern France represent a proximal lateral equivalent of the organic-rich Kimmeridge Clay Formation. The Argiles de Châtillon Formation is composed of two subunits that straddle the Kimmeridgian–Tithonian boundary. Each subunit contains an organic-rich interval. The two conspicuous organic-rich intervals have been linked to either periods of high sea level or greenhouse warming. Here, we use palynology to further understand climate and environmental mechanisms that drove organic matter enrichment. We use bulk organic carbon isotope records (δ13Corg) to correlate the Boulonnais sections with those of the Kimmeridge Clay Formation. The palynological results suggest that the stratigraphically lower organic-rich interval (Kimmeridgian) was deposited under suboxic to anoxic stratified conditions. A large-scale climate shift from cooler/humid to warmer/arid conditions marked the Kimmeridgian–Tithonian boundary, influencing organic matter enrichment in the stratigraphically higher organic-rich interval (Tithonian). In contrast with the lower organic-rich interval, there are no indications of stratified conditions for the higher organic-rich interval. Within this thicker organic-rich interval, cyclic variations in amorphous organic matter distribution, total organic carbon and δ13Corgtrends on a 2 m scale are observed. They co-occur with fluctuations of the palynological assemblages, indicative of more humid versus arid climate conditions, likely alternating on a ∼100 kyr eccentricity timescale. Our results show that under the most humid phases of these overall arid climate conditions, sulfurization of carbohydrates was the dominant control on organic matter preservation. This climate-controlled process that drives organic matter enrichment in the Tithonian can be recognized on a basin-wide scale.

New evidences of subaerial volcanism as a trigger for the Kilian event (Aptian-Albian transition) and major climatic changes from offshore Morocco (DSDP Site 545)

The Aptian-Albian transition is characterized by the presence of four organic-rich intervals all linked with major disturbances of the carbon cycle. Among these, the second subevent of the oceanic anoxic event (OAE) 1b, also called Kilian event, has an Aptian-Albian transition age and is characterized by a negative carbon isotope excursion. On the contrary of the Paquier OAE1b subevent, the Kilian event was not extensively studied and its triggering mechanisms and its consequences on regional climates remain uncertain. Here we present a geochemical (Rock-Eval, Mercury) and mineralogical (clay minerals) study on late Aptian-early Albian sediment cores from the Deep-Sea Drilling Program (DSDP) Site 545 (offshore Morocco, Eastern North Atlantic) where a Kilian level equivalent is present. Mercury (Hg) content results show two main peaks in the late Aptian, immediately below and above the Kilian level equivalent. The absence of correlations between Hg and total organic carbon and Hg and CaCO 3 indicates that the mercury content is mainly related to subaerial volcanic activity, probably issued from the emplacement of the Kerguelen large igneous province in the Indian Ocean. Mineralogical analyses show that the clay content, in our studied interval, mainly reflect the weathering and climatic conditions in Northwest Africa. The evolution in clay mineral assemblages depicts three distinctive climatic intervals: (i) a seasonally contrasted semi-arid climate below the first Hg anomaly; (ii) a moister and warmer climate during the Kilian level equivalent deposition; and (iii) a rapid transition toward more arid climate post Kilian level equivalent deposition that is linked with the second Hg anomaly (large increases in illite and palygorskite). Therefore, our study confirms that the Kerguelen volcanic activity was probably the trigger of the Kilian event and shows that the resulting changes in the carbon cycle caused two major climate changes before and after the Kilian event and a long term aridification of Northwest Africa. • Hg anomalies occurs immediately below and above the Kilian level. • Kerguelen volcanic activity was likely the trigger of the Kilian event • Three distinct climatic periods can be recognized along the late Aptian. • late Aptian represents a turning point for the Cretaceous climate changes.

Water-Based Drilling Fluid Helps Control Extreme Conditions in Gas Shale Play

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper URTeC 208307, “A Water-Based Drilling Fluid for Controlling Deep-Reservoir Extreme Conditions in an Abu Dhabi Gas Shale Play,” by Gabe Manescu, SPE, Schlumberger; Balazs Veer, SPE, TotalEnergies; and Panamarathupalayam Balakrishnan, SPE, Schlumberger, et al. The paper has not been peer reviewed. _ A combination of teamwork and fluids technology proved to be the formula for successfully drilling in high-pressure/high-temperature (HP/HT) conditions for an onshore UAE shale gas play. The success achieved in drilling deep [6200-m measured depth (MD)], long-lateral-displacement (2,000-m) wells with an integrated global operator is a combination dependent on frequent, transparent communication between team members. The environmentally acceptable aqueous drilling fluid delivered a barite sag-free operation in these highly deviated wellbores. Introduction The drilling of unconventional HP/HT wells in the Diyab field can experience difficulty in reaching target depth. One significant challenge is the design of a drilling fluid to manage reservoir pressures and temperatures. The reservoir temperature may reach 165.5°C, requiring mud weights (MW) greater than 20 lbm/gal. Tailoring the drilling-fluid formulation to balance economics and environmental regulations while overcoming HP/HT conditions is critical. In the Diyab drilling campaign, special emphasis was placed on designing the drilling fluid and then on its use in 8½-in. reservoir sections. Background The Upper Jurassic (Oxfordian to Middle Kimmeridgian) Diyab formation, also known as the Dukhan formation, has served as the source rock for several major oil and gas fields in the Middle East. The Diyab formation is unique because of its lower porosities, high carbonate mineralogy, and pressure gradient. Despite the lower porosity, the high-carbonate content defines an extremely brittle target conducive to hydraulic fracture stimulation. Several organic-rich intervals exist throughout the Diyab formation and are separated by less-organic tight limestones, which create different flow units. From a developmental perspective, this condition has the potential to create a high-impact stacked play. The extent of the prospective unconventional Diyab formation within the UAE is limited to the onshore north-central UAE, where most of this area is within Block 1, operated by TotalEnergies. The depth of the prospective basal Diyab interval across Block 1 ranges between 3758 and 4115 m. Three unconventional offset wells (DE-02, DE-03, and DE-04) were drilled by ADNOC within Block 1 to test the productivity of the three submembers of the greater Diyab interval (the Jubaila, Hanifa, and Tuwaiq Mountain formations). After determining that the Hanifa formation had the most-promising productivity, the first TotalEnergies appraisal horizontal well, DE-05, was drilled. After drilling this well, the DE-06 well was the second horizontal appraisal well to be drilled. Additionally, two more deep wells, DE-09 and DE-10, were successfully drilled and evaluated.

Organic matter enrichment of the Late Triassic Bagong Formation (Qiangtang Basin, Tibet) driven by paleoenvironment: Insights from elemental geochemistry and mineralogy

The Upper Triassic dark mudstones in the Qiangtang Basin are key source rocks for petroleum exploration. However, the factors controlling organic matter enrichment in these dark mudstones remain unclear, especially for pro-deltaic core mudstones, which inhibit source rock distribution predictions. Well QZ-7 contains several Upper Triassic dark mudstone intervals (marked by A to G) with total organic carbon (TOC) contents of 0.57–2.73% (avg. of 1.13%). The TOC contents in intervals A, C, E, and G are generally higher than those in intervals B, D, and F. Here, detailed mineralogical and geochemical analyses of these dark mudstones are analyzed to address the organic matter enrichment mechanism. Paleosalinity, redox, detrital influx, sedimentation rate, and productivity proxies suggest that these dark mudstones were deposited in a brackish-salt and anoxic-dysoxic water environment with low detrital influx, sedimentation rate, and primary productivity. Organic-rich intervals of A, C, E, and G generally show higher salinity, lower oxygen levels, detrital influx, and sedimentation rates than the organic-poor intervals of B, D, and F. The significant positive correlations of TOC with paleosalinity proxies and redox indices indicate that anoxic and stratified high-salinity water columns primarily controlled organic matter preservation. The significant negative correlations between TOC and detrital influx proxies and moderate negative correlation between TOC and (La/Yb)N suggest that increasing detrital influxes diluted organic matter, while a low sedimentation rate promoted organic matter preservation. The effect of primary productivity on organic matter enrichment was limited, as evidenced by the weak correlation between TOC and P/Ti.

Subsurface geology and geochemical evaluation of the Middle Jurassic-Lower Cretaceous organic-rich intervals, West Kalabsha area, Western Desert, Egypt

The West Kalabsha (WKAL) area is a western extreme exploratory area of the Faghur Basin in the Western Desert of Egypt. The study of four wells (WKAL; A-1 X, K-1 X, P-1X, and C-1 X), interpreted with twenty seismic lines, shows that the area is dissected by a series of normal faults with an irregular, E–W strike direction. The general trend of the throw is toward the south–southeast. The most prospective area for hydrocarbon (HC) migration is toward the north (upthrown side) of an E-W striking normal fault north of the WKAl-K-1 X and A-1 X wells. The WKAL-P-1 X and WKAL-K-1 X wells in the Faghur Basin were selected for both geochemical evaluation of possible source rock intervals and burial history modelling within the basin. Integration of the wire-line logs with geochemical analysis identified six organic matter-rich intervals (OMRIs) within the Middle Jurassic-Lower Cretaceous sequence. Four intervals are in the Alam El-Bueib-3C (AEB-3C) member with a cumulative vertical thickness of 530ft. The other OMRIs are 110ft in thickness recorded within the AEB-6 (10ft) and Upper Safa (100ft) members. The studied organic matter (OM) reveals type III, IIIC, and IV kerogens (mainly gas-prone) with a terrestrial origin. They have reached a maturity level consistent with the late oil window. The expulsion threshold depth detected in the WKAL-P-1 X well is 12000ft in the AEB-3C member, whereas the active source depth limit (ASDL) is 15000ft in the Safa Member. The present study suggests a paleo-geothermal gradient range between 1.13oF/100ft and 1.39oF/100ft with an estimated regional erosion of 5500 to 7000ft of strata mainly between the Paleozoic-Jurassic and Cretaceous-Tertiary boundaries. The present-day geothermal gradient based on bottom-hole temperatures shows a geothermal gradient of 1.4oF/100ft (WKAL-P-1 X), that increases to the north to 1.65oF/100ft (WKAL-K-1 X). Burial history modelling reveals that sedimentary strata entered the mature oil zone in the Early Cretaceous (110–115Ma) at depths of 7500–8000ft in the deepest part of WKAL-P-1 X and WKAL-K-1 X wells (Paleozoic strata). Maturation continued to present, resulting in Jurassic and Lower Cretaceous strata currently falling into the late oil window.

Heterogeneous characteristics and controlling factors of oil bearing of the Chang 7 shale in the Ordos Basin

Oil-bearing evaluation is the key issue of shale oil geologic exploration, which affects the mobility and effective development of the remaining oil in the shale system. To study the oil content and its controlling factors of shale in Chang 7 member of the Yanchang Formation in the Ordos Basin, we take the core of well H317 as the main research object, analyze the oil-bearing characteristics of shale by conventional rock pyrolysis and multistep pyrolysis experimental methods, and analyze the change characteristics of the elements by handheld X-ray fluorescence. The results indicate that the organic heterogeneity of Chang 7 shale in well H317 is obvious. It can be divided into upper shale and lower shale with 2467 m as the boundary. The upper shale has low total organic carbon (TOC), poor kerogen type, and slightly low oil content, but strong mobility. The lower shale has high TOC, good kerogen type, and good oil content, but poor mobility. The content and type of organic carbon in shale have obvious control effect on oil bearing and mobility. Differences in paleosedimentary environment control the formation of original sedimentary organic matter. During the lower shale deposition period, the climate was humid and the terrigenous input was relatively weak, high paleoproductivity and strong reducing water effectively preserved the original sedimentary organic matter, and finally formed an organic-rich interval with high TOC. Early diagenesis further altered the abundance and type of original organic matter.