Bottom Water Energy Research Articles

Mechanistic Study and Optimization of High Water Cut Phase Management Strategies in Fracture-Vuggy Carbonate Reservoirs with Bottom Water

Fracture-vuggy carbonate reservoirs commonly exhibit sufficient bottom water energy and are characterized by fractures or other interconnected channels with bottom water, which can easily cause high water cut in oil wells during production. The main reasons are as follows: firstly, bottom water coning leads to water flooding. Secondly, artificial water injection flooding fails and shows high water cut in oil production. This paper further clarifies the causes of high water cut in fractured-vuggy carbonate reservoirs and explores rational and feasible management strategies to improve the current water flooding situation and reduce the comprehensive water cut in oil wells. In this paper, numerical simulation technology is used to establish a mechanism model based on the actual reservoir conditions. By comparing the development effects of four bottom water suppression measures, including the adjustment of flow channel, artificial barriers, adjustment of flow potential and gas injection, the optimal measure is selected and the relevant parameters are optimized. Building upon this, further optimization of water injection methods is conducted to explore comprehensive management strategies for enhancing the water flooding effect during high water cut periods. The research demonstrates that bottom water can be effectively inhibited by artificial barriers, leading to an expansion of the swept volume of water injection and achieving the goal of improving the high water cut condition in carbonate reservoirs. Optimal implementation of artificial barriers occurs when the water cut reaches around 80%. Specifically, positioning barriers at the bottom of perforations and setting the barrier radius at 0.15–0.2 times the well spacing obtain the most favorable results. Additionally, short-term injection with long-term shut-in, identified through cyclic water injection simulations, exhibits superior oil recovery enhancement. Validation conducted in a practical field of a fractured carbonate reservoir over a five-year period has led to a substantial increase in oil production by 5.6 × 104 t and a 12% reduction in water cut. The research approach and methodology presented in this paper can provide valuable insights and serve as a reference for the development of high water cut periods in fractured-vuggy carbonate reservoirs.

Water Invasion Prediction Method for Edge–Bottom Water Reservoirs: A Case Study in an Oilfield in Xinjiang, China

Clarifying the water invasion rule of edge and bottom water reservoirs can adjust the reservoir development mode and improve the recovery factor of edge and bottom water reservoirs in a timely manner. Influenced by the size of a reservoir water body, energy intensity and reservoir seepage capacity, the change model of reservoir water influx basically belongs to the exponential growth model of the GM (1,1) model or the self-constraint growth model of the logistic model. The above two models are used to predict and analyze the water inflow of edge and bottom water reservoirs, respectively, and it is found that the change in water inflow of the reservoir with sufficient edge and bottom water energy is more consistent with the prediction results of the GM (1,1) model, but it has a large error compared to the prediction results of the logistic model. The change in water influx in the reservoir with insufficient edge and bottom water energy is consistent with the prediction results of the logistic model and GM (1,1) model. The research shows that the strength of edge and bottom water energy of the reservoir can be determined by analyzing the error of the logistic model in predicting water influx. If we focus on the change in reservoir water influx, the improved GM (1,1) model formed by a Newton parabola interpolation polynomial is used to optimize its background value, which can further improve the prediction accuracy and reduce the prediction error of water inflow of edge and bottom water reservoirs. The method in this paper has certain reference significance for studying the water invasion rule and energy intensity of edge and bottom water reservoirs.

Study on Damage Mechanism of Waterflooding Development in Weizhou 11-4N Low-Permeability Oilfield

Weizhou 11-4N oilfield is a medium-low-porosity and low-permeability reservoir. The oilfield was initially developed by edge and bottom water energy and then transferred to water injection development. Affected by poor physical properties and heterogeneity of the reservoir, the oilfield appeared in the process of water injection development. When the water injection pressure increases, the water injection volume continues to decrease, and it is difficult to meet the injection requirements. On the basis of the analysis of reservoir heterogeneity, void structure, and clay minerals of reservoir, the water injection compatibility experiment, damage evaluation experiment, and nuclear magnetic resonance-velocity sensitivity experiments were carried out to clarify the damage in the process of oilfield water flooding development. Experiments show that the main causes of damage in Weizhou 11-4N oilfield water flooding development process are water quality incompatibility and strong velocity-sensitive damage. The determination of water type shows that the injected water and formation water are MgCl2 water type and NaHCO3 water type, respectively, under the classification of Surin water type, resulting in the formation of scale with calcium carbonate as the main component in the reservoir. Incompatibility of water quality is an important cause of reservoir damage and scaling. In the reservoir-sensitive flow experiment, the experimental core showed strong velocity sensitivity, the average velocity sensitivity damage rate was 466.31%, and the average critical velocity was 2.98 m/d. Nuclear magnetic resonance experiments show that the core has a significant decrease in average pore size after water flooding. The main damage range is the tiny throat of 0-2 μm. In this paper, the main damage interval of velocity-sensitive damage in the Weizhou 11-4N area and the change trend of void structure after velocity-sensitive experiment are clarified by nuclear magnetic resonance and velocity sensitivity experiments. The main cause of block reservoir damage provides the basis for the oilfield to take targeted measures and provides a guarantee for the efficient development of the subsequent oilfield.

Evaluation of the Effectiveness and Adaptability of a Composite Water Control Process for Horizontal Wells in Deepwater Gas Reservoirs

Deepwater gas fields have high bottom water energy and a high risk of seeing water. Higher requirements are put forward for the water control process to control the water effect. This article is based on the actual background and well design of the X gas field in the South China Sea and on three sets of physical simulation experiments and three sets of numerical simulation experiments. An analysis and comparison of the water control effect of a combination of continuous packer, continuous packer and variable density screen tube, and their adaptability evaluation in deepwater gas reservoirs were performed. The results obtained from the numerical and physical simulations are consistent. The experimental results show that the water control process of a continuous packer is mainly based on the water-seeing and water-blocking ability. It is less capable of extending the time to produce water in the horizontal section. However, its water-blocking ability is strong and is able to seal the water spot quickly. It extends the total production time by 12.29% and increases the total gas production by 5.96%; the combined water control process of the continuous packer and variable density screen tube can effectively play their respective advantages of water control. The combination of the continuous packer and variable density screen tube can effectively be advantageous of their respective water control processes, enabling the gas–water interface to advance in a balanced manner, extending the water-free gas recovery period by 11.61%, extending the total gas production time by 15.76%, and increasing the total gas production volume by 13.75%. Both water control processes have good applicability in deepwater gas fields and have certain sand control capability. It is conducive to the one-time completion operation for the commissioning of deepwater gas fields.

Experimental study the flow behaviors and mechanisms of nitrogen and foam assisted nitrogen gas flooding in 2-D visualized fractured-vuggy model

Fractured-vuggy carbonate reservoir played a significant role in unconventional reservoirs, but it was difficult to be developed due to its complex reservoir types. Therefore, it was crucial to find a suitable development program to enhance oil recovery. In this work, a two-dimensional visual physical model of fractured-vuggy reservoir was designed and fabricated according to actual geological data and injection-production characteristics of a representative fractured-vuggy carbonate reservoir, Tahe Oilfield in China. Nitrogen flooding experiment, nitrogen flooding under the high gas energy experiment, nitrogen flooding under strong bottom water energy experiment and foam assisted nitrogen gas flooding experiment were carried out respectively by using two-dimensional visual model. The flow behaviors and the main mechanisms for enhanced oil recovery were revealed by analyzing the oil displacement performance in the visual physical model. Experiment results showed that the gas cap energy and the bottom water energy played a synergistic effect in the nitrogen flooding. The stronger the bottom water energy was, the larger the gas spread area was, and the increase of oil recovery would be very high. If the gas injection rate was too strong, the dominant gas flow channel would be formed prematurely, and the oil recovery was little increase. In addition, foam could effectively control the gas mobility, delay the formation of gas channeling. Meanwhile, the oil recovery of foam assisted nitrogen gas flooding was higher 12% than that of nitrogen flooding. Therefore, the mechanism for improving the degree of development was obtained mainly through gravity differentiation to form the secondary gas top and realize the development of the remaining oil.

Physical simulation of waterflooding development in large-scale fractured-vuggy reservoir considering filling characteristics

The Fractured-vuggy reservoir is one of the world's most important reservoir types and has great exploration and development potential. However, due to the unique geological characteristics of the fractured-vuggy reservoir and limited understanding of its waterflooding development law, it is more difficult to develop. To strengthen the study of the oil-water flow mechanism and the waterflooding development law of fractured-vuggy reservoirs, we built a large-scale 3-D physical model that can reflect the geological morphology and structure of typical well groups. On the basis of the similarity theory, we determined the parameters of the model and its physical simulation. The physical model was applied to the experimental study of the waterflooding development process and the analysis of relevant influencing factors. The results show that the low-high mode can effectively touch the reserves in high parts, and the development effect is better than that of the high-low mode. There is an optimal water injection rate in waterflooding development to maximize the recovery. Weak bottom water energy at the bottom of the reservoir plays a positive role in waterflooding development, whereas strong bottom water energy increases the risk of premature water breakthrough, which is not conducive to waterflooding development. The substantial difference in oil and water viscosities makes viscous fingering more obvious, which reduces the stability of the displacement process and reduces the efficiency of waterflooding development. This study may provide strong guidance to further understand the oil-water flow law and improve waterflooding development efficiency for fractured-vuggy reservoirs.

Experimental study on gas channeling characteristics of nitrogen and foam flooding in 2-D visualized fractured-vuggy model

Fractured-vuggy carbonate reservoirs are becoming the key to oil reserves, but their complex reservoir structure and connectivity make it difficult to be developed. In this work, a visual physical model representing an underground river reservoir in two-dimensional fractured-vuggy reservoir was designed and fabricated to study gas channeling characteristics of nitrogen and foam flooding. In order to further study the influence of the porous media on gas channeling characteristics of foam and nitrogen gas flooding, the visual physical model was filled with 2 mm transparent glass beads and the effects of glass beads on different types of nitrogen and foam flooding gas channeling were studied. Experimental results showed that the synergistic action of gas and water and the bottom water energy were the important factors affected gas channeling. Furthermore, foam could effectively control gas mobility, delay formation of gas channeling. In terms of enhanced oil recovery, the recovery efficiency of foam flooding was 35% higher than nitrogen flooding in the horizontal model. In addition, the glass beads in fractured-vuggy of underground river reservoirs could change the flow direction of the fluid and cause a certain flow resistance, foam was more easily to occur gas channeling in the longitudinal model filled with glass beads. The oil recovery by foam in the longitudinal model filled with glass beads was lower 12% than that in the longitudinal model. Therefore, determined the factors that influence gas channeling had a practical value for enhancing oil recovery in fractured-vuggy reservoirs.

Application of binary compound water control technology in the development of horizontal wells in offshore oil fields

Offshore A oil field is a typical bottom water reservoir with thin thickness of oil layer and large thickness ratio of water layer and oil layer. Due to its strong bottom water energy, serious non-homogenous and the impact of the bottom ridge, most of the horizontal well production segment is seriously water out.Based on interaction relationship between horizontal segment trajectory and oil and-water interface, the distribution and contrast of permeability along horizontal segment and the comprehensive analysis of production performance, this paper combines the characteristics of the residual oil distribution of typical wells and the mechanism of dual compound oil addition, and from the perspective of combining theory with the field creatively put forward the dual composite water control mode of first injecting gel segment plug and then injecting foam. then completed the design of dual compound water control mode and dosage of the typical well. After processing water control measures, the production data shows that the water content decreased significantly after opening the well, from 95.1% before water control to 80%, which was 15% lower. Also the oil production rate has a significant increase from 2.3t/d before water control to maximum to 9t/d after implementing control measures and cumulative oil increase is 550t with effective period of 135 days. The effect of oil and precipitation is obvious.

Magneto-biostratigraphic age models for Pleistocene sedimentary records from the Ross Sea

Abstract The Ross Sea is one of the major sites of formation for Antarctic Bottom Water (AABW), a key component of the global ocean overturning circulation. However, there is currently a lack of high quality stratigraphic records documenting how this water mass flows from the continental shelf into the abyssal ocean, and specifically how this pathway is affected by changes in ice sheet cover on the Ross Sea continental shelf through Pleistocene glacial-interglacial cycles. Over the course of two expeditions in 2015 and 2016, a suite of cores from the upper continental slope to the abyssal plain was obtained by the Korea Polar Research Institute from the R/V Araon. The age of these cores ranges from Holocene to the latest Pliocene, and they hold the potential to document a source-to-sink record of AABW transfer into the abyssal Pacific Ocean. The cores are located in regions with distinct differences in bottom water energy, with high-energy cascading water masses on the upper slope creating the potential for erosional hiatuses, passing downslope into a lower energy setting. To decipher the complex environmental records and allow core-to-core correlation, robust chronostratigraphies are essential. Here, we present age models for four of these cores, based on correlation between their magnetostratigraphy and the geomagnetic polarity timescale, resulting in sedimentation rates between 1.5 cm/ky and 0.5 cm/ky. Rock magnetic data indicate the remanence is carried by magnetite with an almost ubiquitous contribution of high coercivity fraction that is not demagnetised by 100 mT. We demonstrate that a reliable magnetostratigraphy is established for each of these cores, and magnetic properties can be used to identify potential hiatuses in the cores and as a proxy for sedimentary grain-size.

Experimental study on waterflood development in large–scale karst structures

On the basis of the seismic data volume of a certain well group unit of the Tahe fracture–vuggy reservoirs, the similarity theory is adopted, and a numerical model with geometric, kinematic, dynamic, and fracture–cavity characteristic parameter similarities is designed. Moreover, a large–scale three–dimensional physical karst cave model is built with the carving technology. The model is used in the experimental study of two–phase flow of oil/water during waterflood development and in the analysis of relevant influencing factors. Results show that the displacement efficiency of low–high mode is obviously superior to the case of high–low mode. A high crude oil viscosity is found to equate to a low ultimate recovery. High–velocity water flooding improves ultimate recovery but results earlier water breakthrough. Well shutdown improves recovery factor and well shutdown during middle water cut stage achieves optimum effects. Bottom water energy benefits oil reservoir exploitation to some degree.

Experimental study on lateral flooding for enhanced oil recovery in bottom-water reservoir with high water cut

The regular enhanced oil recovery (EOR) methods in bottom-water reservoir aim to increase the vertical sweep efficiency. After multicycle chemical injection, the injected slug cannot effectively control the bottom water coning in the presence of fully developed water channel, decreasing the sweep efficiency and lowering economic efficiency. To improve the development effect of bottom-water reservoir, lateral flooding is proposed as a more cost-effective EOR technique by displacing the oil formation horizontally. In this study, three lateral flooding tests were performed in a three-dimensional physical model based on the geometric similarity criterion. Bottom-water reservoirs were simulated by sandpacking oil and water formations according to the parameters of the target oil reservoir. The bottom-water energy was supplied by an ISCO constant pressure, and the lateral flooding was conducted by an ISCO constant rate pumps. The oil recovery, water cut, pressure drop, and saturation variation maps obtained from these tests were recorded and analyzed. A comparison of these results between different crosslinked polymer slug tests shows that the improved oil recovery by lateral waterflooding can be mainly attributed to a significant increase in horizontal sweep efficiency. After 0.3 PV crosslinked polymer injection, about two-thirds of that was used to shut off the main water channels; the chemical packer, formed by the surplus polymer spreading along the oil/water interface under the drive of lateral injection, can inhibit bottom water from coning into the oil formation and prevent the crossflow. The 0.2, 0.3, and 0.4 PV crosslinked polymer injection can increase oil recovery by as much as 11.35, 36.23, and 39.74% of the original oil-in-place (OOIP), respectively. This confirms that lateral flooding is an efficient EOR method in bottom-water reservoir.

Experimental study on feasibility and mechanisms of N2/CO2 huff-n-puff in the fractured-cavity reservoir

The purpose of this experimental study is to evaluate the efficiency and performance of gas huff-n-puff in the single well on recovery of light oil from a multi-well fractured-cavity reservoir and to reveal the feasibility of gas mediums huff-n-puff. In order to reveal the production performance characteristics and mechanisms of gas huff-n-puff on activating the remaining oil, a visualized and pressure resistant two-dimensional (2D) fractured-cavity model and a three-dimensional (3D) multi-wells model were designed and fabricated respectively based on the similarity theory. With the simulated oil and brine reservoir samples in Tahe oilfield, five groups of tests in 3D model and five groups in 2D model were performed, each of which included bottom water energy depletion driving, gas injection stage, soaking stage, and gas puff stage. The production performances of N2, CO2, and N2/CO2 mixture huff-n-puff in the 3D model were firstly analyzed. Then the remaining oil recovery principles of each gas were compared and analyzed in the 2D model. Finally, the influence of gas injection position was studied both in the 2D and 3D models. The experiment results from 3D model demonstrated that higher oil recovery factor could be achieved through N2 huff-n-puff which had shown enormous advantages than CO2 and N2/CO2 mixture. Furthermore, the results from 2D model indicated that there were four types of the remaining oil, all of which could be recovered through N2 huff-n-puff if it was implemented in the relative low-position well. The results from experiments on gas injection position conducted in the 3D physical model demonstrated the better oil recovery effect if N2 was injected in low-position well which was consistent with the results concluded in the 2D model. This paper confirmed the effectiveness and advantages of N2 huff-n-puff compared with CO2 and gas mixture in the light oil-saturated fractured-cavity reservoir.

Large-scale paleoceanographic variations in the western Mediterranean Sea during the last 34,000 years: From enhanced cold-water coral growth to declining mounds

Abstract Quantitative and qualitative analyses of cold-water coral (CWC) fragments from two sediment cores obtained from the Melilla Mounds Field (MMF) in the Alboran Sea, western Mediterranean Sea, reveal an alternation of periods dominated by distinct CWC species. The lower parts of the cores are dominated by the CWC species Lophelia pertusa, which is successively replaced in the upper parts by the species Madrepora oculata and Dendrophyllids. The transition in the macrofauna coincides with a characteristic change in the benthic foraminiferal assemblage. Benthic foraminiferal assemblage BFAglacial, in accordance with benthic (Cibicides lobatulus) and planktic (Globigerina bulloides) δ13C and δ18O values provide evidence for generally high surface productivity, cold and well-ventilated bottom waters lasting from the end of Marine Isotope Stage 3 (33.3 ka BP) reaching a maximum at the transition Allerod–Younger-Dryas. Together with δ13C of the organic carbon and Rock-Eval pyrolysis, benthic foraminiferal assemblage BFAinterglacial established since the Early Holocene indicates that the MMF experienced a decrease in bottom-water energy that caused an organic carbon-enrichment in the sediments and also depleted oxygen waters. Compared to the pre-Holocene interval dominated by the planktic foraminifera Neogloboquadrina incompta and the benthic foraminiferal assemblage BFAglacial, the organic carbon deposited during the Holocene in the MMF contains more refractory components in relation to sea-level rise and modern oceanographic configuration. Based on our data, we suggest that L. pertusa has a higher ecological requirement than M. oculata and Dendrophyllids especially with regard to oxygen and nutrient availability.

Mechanism and influencing factors of EOR by N2 injection in fractured-vuggy carbonate reservoirs

Multi-scale fractures and karst caves are the main reservoir bodies in fractured-vuggy carbonate reservoirs, and there are still various types of remaining oil after water flooding. Oilfield development practices demonstrate that the method of N2 injection can enhance oil recovery effectively. However, the development effect of a single well with gas injection is highly different, and the mechanism and influencing factors of gas injection for enhanced oil recovery are not clear. To reveal the mechanism and influencing factors of enhanced oil recovery through physical experiments and numerical simulation, we have produced a series of physical models of fracture and cave combinations based on the characteristics of this type of reservoir, established a visual physical simulation experimental apparatus and process, and analyzed the main factors affecting the production efficiency of gas injection wells in combination with the actual development of oilfields. The research results demonstrate that the primary types of remaining oil after water flooding in fractured-vuggy carbonate reservoirs are oil at the top of karst caves, oil shielded from high flow channels and oil not swept by the wells. The mechanism of gas injection-enhanced oil recovery is primarily composed of three aspects. First, the injected gas forms a secondary gas cap because of the effect of gravitational differentiation, and the gas cap can displace the remaining oil on top of the karst cave that the injected water cannot drive; Then, injected gas can change the pressure field distribution and the direction of fluid flow after water injection, driving the remaining oil that is shielded by high flow diversion channels; in addition, the injected gas has a certain effect on the reservoir energy supply. The geological conditions, such as the remaining reserves of injecting gas wells, filling level, well-reservoir position relationship and bottom water energy, have great influence on the development effect. The starting time of gas injection has an optimal point, and injecting too early or late will affect the oil recovery. The cyclic gas injection quantity and gas injection velocity are the primary injection production parameters that influence the production effect of a gas injection well. Finally, the principle of selecting a well for gas injection according to the research results is discussed, which can provide reference for selecting gas injection wells and injection methods of fractured-vuggy reservoirs in Tahe Oilfield.

Long-term coastal openness variation and its impact on sediment grain-size distribution: a case study from the Baltic Sea

Abstract. We analysed the long-term variations in grain-size distribution in sediments from Gåsfjärden, a fjord-like inlet in the southwestern Baltic Sea, and explored potential drivers of the recorded changes in the sediment grain-size data. Over the last 5.4 thousand years (ky) in the study region, the relative sea level decreased 17 m, which was caused by isostatic land uplift. As a consequence, Gåsfjärden was transformed from an open coastal setting to a semi-closed inlet surrounded by numerous small islands on the seaward side. To quantitatively estimate the morphological changes in Gåsfjärden over the investigated time period and to further link the changes to the grain-size distribution data, a digital elevation model (DEM)-based openness index was calculated. The largest values of the openness indices were found between 5.4 and 4.4 cal ka BP, which indicates relatively high bottom water energy. During the same period, the highest sand content (∼ 0.4 %) and silt / clay ratio ( ∼ 0.3) in the sediment sequence were also recorded. After 4.4 cal ka BP, the average sand content was halved to ∼ 0.2 % and the silt / clay ratio showed a significant decreasing trend over the last 4 ky. These changes were found to be associated with the gradual embayment of Gåsfjärden, as represented by the openness indices. The silt / clay ratios exhibited a delayed and relatively slower change compared with the sand content, which indicates different grain-size sediment responses to the changes in hydrodynamic energy. Our DEM-based coastal openness indices have proved to be a useful tool for interpreting the temporal dynamics of sedimentary grain size.

Type Curves for a Reservoir Subject to Active Bottom Water Drive

The use of dimensionless pressure and dimensionless pressure derivative type curves has fully overcome the challenges experienced in the use of straight line methods and has brought about major successes in well tests analyses. Flow periods and reservoir boundary types are easily delineated and identified with the use of these curves. Furthermore, near wellbore characterization results are now more reliable. In this study, type curves for a reservoir subject to bottom water energy and a vertical well completion are developed to reveal specific signatures that can be used to achieve efficient pressure test analysis. Both early and late flow periods were considered for a wellbore of negligible skin and wellbore storage influences. Results obtained show that dimensionless pressures depart from infinite-acting behavior and attain steady state at dimensionless time of order proportional to the square of dimensionless reservoir thickness. Wellbore dimensionless radius affects dimensionless time of attainment of steady state inversely, which is rather accelerated by large fluid withdrawal rates (large pressure drawdown). On the other hand, dimensionless pressure derivatives show gradual collapse to zero after expiration of infinite flow. The rate of collapse is strongly affected by wellbore properties and pressure drawdown. Radial flow is generally characterized by a constant slope of 1.151 during which period the dimensionless pressure derivative gave a value of 0.5. Following assumption of negligible wellbore skin and storage, no early time hump is observed on dimensionless derivative curves.

Macrobenthic community structure in a high-productivity region: Upper Campanian Mishash Formation (Israel)

A paleoecological study of molluscan fossils from the Upper Campanian Mishash Formation of southern Israel was undertaken to provide new insights into the inference of bottom-oxygen levels, food availability and bottom water energy on high-productivity seafloors. Quantitative data for molluscan analysis were collected from each of the main lithologies that comprises the high-productivity facies-mosaic of the Mishash sea, i.e., organic-rich carbonates, thin-porcelanite and -chert beds, and loosely- to densely-packed shell beds. Diagenesis has obliterated faunal remains in carbonate layers. Pollution studies in modern environments have demonstrated the close connection between stressful conditions – such as organic load, toxicity and water and sediment aeration – and benthic community structure, as expressed through species richness, evenness, taxonomic composition, body sizes, and trace fossil assemblages. These connections and behavior patterns were investigated in the varied benthic fossil assemblages of the Phosphate Member of the Mishash Formation. Bivalves and gastropods dominated the macrofauna in all rock types but in varying abundance and taxonomic compositions, changing from diverse assemblages (15–22 species per sample) to less rich ones (5–7 species) and even barren intervals. This range of abundance neatly describes the lateral mosaic and heterogeneous nature of the upwelling environment of the Upper Campanian and is comparable to the mosaic of sub-facies that characterize modern upwelling facies-belts today (e.g., in the Benguela upwelling system). The variations shown for the macrobenthic assemblages within the Mishash Formation high-productivity regime are associated with proximity to the high-productivity center or to periods of intensified upwelling. The inferred, variable oxygen levels and organic-load changes point up how dynamic these regions really are. This is expressed in community structure and composition, which vary between two extremes: The low-oxygen biofacies 1 is characterized by a smaller range of shell sizes, infaunal deposit feeders and low species richness, while the high-oxygen biofacies 3 is characterized by greater ranges in shell size, infaunal but also epifaunal suspension feeders and higher species richness. The chert and porcelanite beds exhibit a variety of sub-environments and seem to represent an intermediate facies that bridges the two extremes of biofacies 1 and 3. The above ecological criteria may provide evidence of the original carbonate environment for the thin-chert and -porcelanite layers. The chert or porcelanite samples that contain a moderately even and rich assemblage of species intermediate to the values of the two end-member facies are taken to represent a shallow-water carbonate marine environment marginal to the upwelling center, whereas layers of chert and porcelanite containing very low species richness and evenness are taken to characterize primary depositional environments proximal to the paleo-upwelling cells.