Paleokarst System Research Articles

Karst control on reservoir performance of a developed carbonate geothermal reservoir in Munich, Germany

The Upper Jurassic ‘Malm’ carbonates of southern Germany are currently the most developed reservoir for geothermal energy production in Germany. Although many studies have investigated the Malm carbonate reservoir, few studies have focused on the palaeokarst system – despite the fact that the reservoir is frequently referred to as ‘Malmkarst’. Data from 25 wells in the Malm reservoir, including well logs, seismic data and hydraulic information, systematically demonstrate the presence of karstification. This is apparent in the form of large sinkholes and amplitude anomalies on the seismic scale, as well as caves observed in the image log and caliper log data. Flowmeter logs correlated with the karstified section on the image log reveal thin zones of elevated permeability, contributing most of the flow at the wellbore scale. The comparison between measurements from core data and well tests provides evidence of the reservoir's excess permeability, exhibiting values several orders of magnitude higher than those measurable in cores. The stratigraphic record suggests that karstification occurred during the Jurassic and Cretaceous, prior to the burial of the carbonates by the sediments of the alpine molasse basin.

Examples of Paleokarst in Mesozoic Carbonate Formations in the Carpathian Foreland Area

A paleokarst system developed in the Upper Jurassic–Lower Cretaceous carbonate complex has been recognized in the Carpathian Foreland area. Well logs testing and core data as well as an acoustic imager, a microresistivity scanner and six-arm dipmeter images were used to identify and analyze the character of the paleokarst features. A detailed interpretation of microresistivity and acoustic image logs allowed for the identification of different types of karst forms, such as caverns; multidirectional fractures, including fractures widened by dissolution; and the type of sediments filling them. The analysis of the seismic survey was conducted by linking the paleokarst characteristic features recognized in the seismic image to the karst intervals determined from borehole data. The set of seismic attributes calculated from the analyzed 3D seismic data, including the RMS amplitude, instantaneous frequency, consistent dip, variance, sweetness and relative acoustic impedance, helped to delineate the zones of the paleokarst distribution. Within the interpreted paleokarst surface developed in the carbonate formations in the study area, there are sinkholes, limestone pavements and valleys. Furthermore, in the northwestern part of the analyzed area, the development of paleokarst forms is related to the presence of a relatively deep branch of a paleovalley formed in the Paleogene, as well as to numerous discontinuities developed in carbonate formations. The development of this type of larger karst form was probably controlled primarily by tectonic faults. The research conducted by the authors of this paper showed the widespread presence of paleokarst features in Upper Jurassic–Lower Cretaceous carbonate formations in the study area. A good spatial identification of the paleokarst surface can be important in a regional context, since the highest part of the profile of carbonate formations is the most important reservoir for geothermal or hydrocarbon resources in this region.

Genesis, Distribution, and Characterization of a Paleokarst Subsurface River System in the Tahe Area, Tarim Basin, Western China

Summary Subsurface river systems constitute one of three major paleokarst types that make up Ordovician reservoirs in the Tahe area of the Tarim Basin. The total length of the river system is approximately 400 km, and the reserves associated with this karst type are more than 200 million tons. However, it is manifested that 47% of drilled wells have not encountered river paleokarst, while 50% of wells that have encountered river paleokarst are fully filled due to the poor understanding of the paleokarst of this region, resulting in a significant variation of production capacities. In this study, we propose a detailed data integration approach with outcrops, drilling, logging, seismic profiles, and dynamic data to delineate the complex paleokarst river system in the Tahe area. The karst geological theory with reservoir characterization is combined in particular. The workflow of clarifying the main controlling factors, architecture types, and development distribution modes of the subsurface river system is established. Fill material type, sequence of fill structure, and fill controlling factors are also revealed. A quantitative characterization method of the subsurface rivers is established adopting predictions based on seismic data and high-resolution geostatistical and geological modeling. The Ordovician reservoirs in the Tahe area comprise three paleokarst river systems with different characteristics. Karst paleogeomorphology is the main control over the overall flow direction and plane distribution of the subsurface rivers. Changes in the surface of the phreatic zone are crucial in controlling the vertical layers and scale of the rivers. The combined action of faults plays a decisive role in controlling the anastomosing pattern of the rivers. Single-branch channels, reticulated channels, and structural corridors in single-layer or multilayer styles are the main subsurface river types. Trunk channels, branch channels, hall caves, and inlets/outlets are dominant structures in the architecture of the river system. Sand-mud, breccia, and chemically precipitated materials are the most common fill types. Three typical sequences of fill structure and four spatial combination modes exist in the subsurface river system. The morphology and fill characteristics of rivers are predictable using seismic attributes, such as frequency division energy, frequency division inversion, and coherent energy gradient. 3D models are constructed by multivariate control multipoint geostatistical method, which can characterize the strong heterogeneity characteristics of subsurface river systems. This complex paleokarst system enables remarkable results for the adjustment of the reservoir development plan through quantitative characterization.

Seeing beyond the outcrop: Integration of ground-penetrating radar with digital outcrop models of a paleokarst system

Paleokarst breccias are a common feature of sedimentary rift basins. The Billefjorden Trough in the High Arctic archipelago of Svalbard is an example of such a rift. Here the Carboniferous stratigraphy exhibits intervals of paleokarst breccias formed by gypsum dissolution. In this study we integrate digital outcrop models (DOMs) with a 2D ground penetrating radar (GPR) survey to extrapolate external irregular paleokarst geometries beyond the 2D outcrops. DOMs are obtained through combining a series of overlapping photographs with structure-from-motion photogrammetry, to create mm- to dm-resolution georeferenced DOMs. GPR is typically used for surveying the shallow subsurface and relies on detecting the contrasts in electro-magnetic permittivity. We defined three geophysical facies based on their appearance in GPR. By integrating subsurface geophysical data with DOMs we were able to correlate reflection patterns in GPR with outcrop features. The chaotic nature of paleokarst breccias is seen both in outcrop and GPR. Key horizons in outcrop and the GPR profiles allow tying together observations between these methods. Furthermore, we show that this technique expands the two-dimensional outcrop surface into a three-dimensional domain, thus complementing, strengthening and extending outcrop interpretations.

Electrofacies classification of deeply buried carbonate strata using machine learning methods: A case study on ordovician paleokarst reservoirs in Tarim Basin

The paleokarst system is one of the main carbonate reservoirs, which can form important super-large oil fields. There are many typical paleokarst reservoirs in the Tarim Basin Ordovician strata, mainly composed of caves, vugs, and fractures. Due to the deep burial depth and strong heterogeneity, qualitative identifying the different scale fracture-vuggy reservoirs from the tight limestone around the wellbore is a real challenge in the industrial community. In this paper, machine learning methods were used to classify electrofacies. Firstly, core samples and electrical imaging logging of the paleokarst reservoirs are observed in detail and a core-electrical imaging chart was established. Secondly, conventional logging data was optimized and preprocessed for data mining, using Principal Component Analysis (PCA) algorithm and K-means algorithm. High-resolution electrical imaging logging was chosen as a constraint to recognize electrofacies, and an electrofacies-lithology database was established. Thirdly, based on the electrofacies-lithology database, Linear Discriminant Analysis (LDA) algorithm was used to build an electrofacies prediction model, which can automatically identify the electrofacies in carbonate strata, with a coincidence rate of 92.2%. Finally, the model was used to quantitatively recognize paleokarst reservoirs and their distributions. The electrofacies machine learning workflow proposed in this paper could be used in Tarim Basin and other similar paleokarst reservoirs, which can improve exploration efficiency and save economic cost.

Three-Dimensional Geophysical Characterization of Deeply Buried Paleokarst System in the Tahe Oilfield, Tarim Basin, China

Paleokarst reservoirs are the major type of the Ordovician carbonate reservoirs in the Tahe Oilfield. Due to the strong heterogeneity in distribution, it is a real challenge to detect the spatial distribution of paleokarst reservoirs, especially those deeply buried more than 5500 m in the Tahe area. Based on the abundant core samples, this paper first described the structure of paleocaves drilled by well. Second, after time–depth conversions, the results from drilled wells were tied to three-dimensional (3D) seismic datasets, and then the threshold of host rocks and caves in wave impedance were identified. Third, the seismic-scale mapping and visualization of the paleokarst reservoirs were achieved by tracing the distribution of paleocaves. This approach was applied in the well T403 area, and the structure of the paleokarst, especially the runoff zone, was interpreted. 3D structure and spatial distribution of the paleokarst system was demonstrated by plane, vertical, and 3D models. Additionally, according to the hydrology genetic relationships, the paleocaves in the runoff zone were divided into sinkholes, main channel, and branch channel. The approach of a 3D geophysical characterization of a deeply buried paleokarst system can be applicable to Tahe and other similar paleokarst oilfields, which will guide hydrocarbon exploration in paleokarst reservoirs.

Multiscale geological-geophysical characterization of the epigenic origin and deeply buried paleokarst system in Tahe Oilfield, Tarim Basin

Abstract The irregular structures of paleokarst systems are among the major factors resulting in carbonate reservoir heterogeneity, which restricts the further exploration of paleokarst reservoirs. Using multiscale geological-geophysical (G&G) data, including thin-section, core-sample and well-log data, the caves, cave fillings and fractures in individual wells in the northern Tahe Oilfield were characterized. Guided by seismic datasets, the interwell paleokarst structures were interpreted in profile and plane view. The paleokarst system in the study area was divided into four vertical zones, and the relative genetic structures were classified in the upper three zones. The epikarst zone includes paleo-canyons, karst towers, dolines, paleo-soils, and fractures, and the water flows horizontally. The vadose zone includes vadose caves and fractures, and the water flows vertically. The epiphreatic zone includes chambers, main channels, branch channels and fractures, and the water flows primarily horizontally along the water table. For each genetic type, the total hydrocarbon production and monthly production were analyzed. Furthermore, a genetic classification model was built for the paleokarst reservoirs of the study area, which increased our understanding of the structure of the paleokarst system. The results of the structure classification and production analysis can effectively guide hydrocarbon exploration, and this approach can be applied to similar paleokarst reservoirs.

THREE‐DIMENSIONAL SEISMIC GEOMORPHOLOGY OF PALEOKARST IN THE CRETACEOUS MACAÉ GROUP CARBONATES, CAMPOS BASIN, BRAZIL

Three‐dimensional seismic data from an oilfield in the Campos Basin, SE Brazil, was used to characterize a karstified unconformity surface at the top of the Albian – Cenomanian Macaé Group. Macaé Group carbonates underwent intense karstification associated with subaerial exposure during a period of some 10 to 15 Ma resulting in the development of canyons, valleys, sinkholes and cave systems. The carbonates host commercial oil accumulations at a number of important oil fields. Understanding how the karstification process has affected the carbonates' reservoir properties is essential for future exploration, not only to improve recovery rates but also to avoid drilling‐related issues such as thief zones. In this context, this study aims to characterize the top‐Macaé Group paleokarst system by investigating the morphology of the associated reflectors recorded on seismic data, together with endokarst features in the underlying carbonate succession such as cave systems and collapse structures. The top‐Macaé Group seismic horizon can be divided into two principal geomorphological domains: highlands, characterized by abrupt relief with well‐developed erosional features; and lowlands, marked by a smoother topography. Collapse sinkholes occur in both domains and take the form of closed circular depressions. The study of endokarst features from an analysis of amplitude anomalies (bright spots) indicates the presence of heterogeneous cave systems. The interpreted data contributes to a better understanding of the spatial distribution of the paleokarst system in the Macaé Group carbonates and may assist with future hydrocarbon exploration in the Campos Basin.

Unravelling the origin of a Paleogene unconformity in the Latium-Abruzzi carbonate succession: A shaved platform

The Latium-Abruzzi carbonate platform constitutes the sedimentary record from the Triassic to the late Miocene of the southern Tethyan margin outcropping in the Apennines. This sedimentary succession is punctuated by an important hiatus between the end of the Cretaceous and the early Miocene. This Paleogene hiatus has long been interpreted as the result of prolonged subaerial exposure. However, no evidence of such exposure has ever been documented on the Latium-Abruzzi platform. This is contrary to the coeval and adjacent Apulia and Lessini carbonate platforms where a broadly-developed paleokarst system formed between the Eocene and the early Miocene. The assumption that the Miocene marine transgression deeply eroded a karstified Cretaceous substrate on the Latium-Abruzzi platform is not supported because this surface appears in the field as a perfectly flat, bioeroded paraconformable surface. In this work, a marine origin is proposed for this unconformity. From late Paleocene to early Miocene, the Latium-Abruzzi carbonate platform was a shaved isolated platform exposed to wave action in the middle of the proto-Mediterranean area. Bioclastic sediment accumulated during transgressive and sea-level highstand phases, whereas in the following falling and lowstand stages, sediment was eroded as the seafloor came into the zone of wave abrasion. The sediment eroded from the platform was shed into the basin where it produced coarse bioclastic intercalations in the hemipelagites of the Scaglia Formation. At the end of the Oligocene, when the adjacent basin was filled, a small ramp was established in the transitional zone between the platform and the basin. There, cross-bedded carbonates accumulated in middle to outer ramp environments. The inner ramp remained within the zone of wave abrasion such that sediment generated there was washed out into the middle and outer ramp environments. Finally, in the early Burdigalian, the Mediterranean progressively evolved into closed sea during the initial stages of the closure of Indo-Pacific connection. The reduction of the wave-base depth, typical of an enclosed sea, created increased accommodation space allowing sediments to accumulate and form the complete inner to outer ramp facies belts of the Miocene Latium-Abruzzi ramp.

Paleokarst system based on FMI facies analysis: a case study in the Yingshan Formation at the Tazhong uplift in the Tarim Basin, Northwest China

FMI (Formation Micro-Scanner image) is a widely used technique to analyze reservoir properties with the advantages of being high resolution and providing a visible and quantifiable image. In this study, eleven FMI facies are classified based on integrating borehole images, cores, thin sections, and well logs. The facies include such reservoirs as caves and tectonic fractures, sedimentary interbeds, such as high-resistivity facies, and two facies of fractures and holes. In addition to the description of karst systems in the borehole images, anomalies in the GR (gamma ray) well log curve may complement the results from the FMI images. The GR curve shows that the composite facies of fractures and holes could be an effective reservoir instead of caves with high GR values because the holes filled with oil and gas are connected by fractures. Two suites of karst systems formed in the SQ4, while small scale caves and holes formed in the SQ3 because of their lithological and structural locations. The results of this study demonstrate that unconformities and tectonic geomorphology control the differences between the paleokarst systems.

Paleokarstic macroporosity development at platform margins: Lessons from the Paleocene of north Spain

Platform margins have been targets for carbonate exploration because they are commonly seismically resolvable and reservoir prone for several critical reasons including karstic porosity. Platform margin karst models, mainly based on the Quaternary of the Caribbean (tropical humid, including “flank margin caves”) are well documented, but analogs remain rarely identified in the stratigraphic record. Analysis of a remarkably well-exposed macroporosity paleokarst system from the Paleocene of north Spain, formed under an arid to semiarid climate, provided a model that differs from the Caribbean template. Implications for exploration and appraisal strategies, include provisos regarding (1) how early fracture intensity can be estimated at platform margins, (2) the absence of flank margin caves at the margin, (3) the absence of karstic features at or immediately below the main lowstand surface, (4) the presence of cave-free corridors such that the porosity zones could be missed completely, and (5) the stratigraphic inheritance of caves into successive levels adjacent to the cave-free zones. Quantitative data were evaluated on the main macroporous intervals. We also compared this model with other paleokarst models in the literature to emphasize the diversity of approaches that can be used to evaluate paleokarst targets: “one size does not fit all.”

Paleokarst system development in the San Andres Formation, Permian Basin, revealed by seismic characterization

Paleokarst systems are one of the major factors resulting in carbonate reservoir heterogeneity and compartmentalization. Nevertheless, few effective workflows have been proposed to map the 3D distribution of such systems. We describe a detailed seismic characterization approach integrating core, well log and rock physics analysis, to reveal a complex subsurface paleokarst system in the San Andres Formation, Permian basin, West Texas. In the area of high volume production, the collapsed paleokarst system is characterized by irregularly developed crackle and fracture breccias, mosaic breccias and cave fillings in the Upper San Andres Formation, which are delineated using seismic acoustic impedance. Along the transition from platform to basin, the paleokarst system is marked by a linear collapse including sags and small vertical faults that are recognizable in seismic imaging. Production data indicates that tight paleokarst zones cause reservoir compartmentalization and influence fluid communication between wells. The complex paleokarst system development is explained using a carbonate platform hydrological model, an outcrop analogue similar to modern marine hydrological environments within carbonate islands. Our method of model development for complex subsurface paleokarst systems may be applicable to other paleoenvironments.

Characterizing seismic bright spots in deeply buried, Ordovician Paleokarst strata, Central Tabei uplift, Tarim Basin, Western China

Anomalous seismic-amplitude bright spots are a common feature in deeply buried (5500 to 6500 m) Ordovician limestone strata in the Central Tabei Uplift area of the Tarim Basin in northwest China. Those anomalies have proven to be useful indicators of reservoir quality. The bright spots as seen on seismic data are tied to high-gamma ray, low-velocity zones in wireline logs, and correspond to clastic cave sediment-fills in the host limestone in core. Synthetic seismic models confirm this relationship between seismic bright spots and cave-sediment fills. A seismic traveltime map of the top Ordovician unconformity illustrates erosional topography and seismic geomorphologic patterns associated with the unconformity with numerous sinuous fluvial channels and canyons, fluvial valleys, sinkholes, and tower karsts and hills. A mature surface drainage system interacted with a near-surface karst system and allowed terrigenous sediments to enter an underground cave system. Karst-related bright spots probably correspond to paleocaves that initiated along an early regional fracture network and later were enhanced and altered by additional discharges in the surface drainage system. A few examples of bright spots are interpreted to be related to postkarstification faults that might have had a hydrothermal origin. Distribution of bright spots provides a useful reference in mapping regional collapsed-paleokarst systems. Bright spots typically are associated with circular and linear faults, and V-shaped depression patterns are related to a collapsed paleocave complex. Seismic-scale mapping and visualization of the paleokarst system can be achieved by interpreting circular and linear faults using continuity attributes and by analyzing the relationship among faults, local depressions, and bright spots.

Floor-age constraining of a tectonic paroxysm of the Pyrenean orogen. Late Middle Eocene mammal age of a faulted karstic filling of the Quercy phosphorites, south-western France

Breaking effects of a Pyrenean compressive tectonic phase are recognised in northern Pyrenean foreland, such as in the Quercy Jurassic limestone platform. The age of this phase still remains poorly constrained. In the so-called Quercy phosphorites area, within the Prajoux–Mémerlin paleokarst system near the city of Cajarc, a clay filling shows a strike slip fault effect, close to N–S direction. According to an analysis of the mammal fauna and inferred chronology of the polyphased paleokarst infilling, the age of the faulted clay is late Middle Eocene, with a mammal age estimate of 41.5 Ma. This provides reliable precise floor-age constraining for the involved sismo-tectonic paroxysm, seemingly related to the Pyrenean tectonic compressive phase.

Floor-age constraining of a tectonic paroxysm of the Pyrenean orogen. Late Middle Eocene mammal age of a faulted karstic filling of the Quercy phosphorites, south-western France

Breaking effects of a Pyrenean compressive tectonic phase are recognised in northern Pyrenean foreland, such as in the Quercy Jurassic limestone platform. The age of this phase still remains poorly constrained. In the so-called Quercy phosphorites area, within the Prajoux—Mémerlin paleokarst system near the city of Cajarc, a clay filling shows a strike slip fault effect, close to N-S direction. According to an analysis of the mammal fauna and inferred chronology of the polyphased paleokarst infilling, the age of the faulted clay is late Middle Eocene, with a mammal age estimate of 41.5 Ma. This provides reliable precise floor-age constraining for the involved sismo-tectonic paroxysm, seemingly related to the Pyrenean tectonic compressive phase. © 2000 Editions scientifiques et médicales Elsevier SAS

Multiple Episodes of Dolomitization in the Arbuckle Group, Arbuckle Mountains, South-Central Oklahoma: Field, Petrographic, and Geochemical Evidence

ABSTRACT The Cambro-Ordovician Arbuckle Group in the Arbuckle Mountains, south-central Oklahoma, had a complex history of dolomitization that resulted in two different geometries of dolomite bodies: stratal dolomite of stratigraphically consistent, widespread distribution, and non-stratal dolomite of stratigraphically inconsistent, local occurrence. Stratal dolomite includes the Royer and Butterly units in the lower Arbuckle Group. Most stratal dolomite samples are coarsely crystalline and have 87Sr/86Sr ratios (0.70890-0.70919) similar to Late Cambrian limestone and coeval seawater (0.7089-0.7092). All stratal dolomite and Arbuckle limestone samples have low 18O values (- 5.3 to - 11.9). The cross-platform distribution of stratal dolomite, coupled with dominant 87Sr/86Sr ratios similar to coeval seawater, suggests that stratal dolomite formed during early diagenesis. However, the low 18O values indicate that all stratal dolomite and Arbuckle limestone samples suffered postdepositional modification, probably as the result of infiltration of meteoric water during development of the Ordovician unconformities. Nonstratal dolomite is present in two areas: the Tishomingo Anticline and the Arbuckle Anticline. In the Tishomingo Anticline area, massive bodies (> 10 km2) of nonstratal dolomite are present in a paleokarst system of pre-Middle Ordovician age. Some of the nonstratal dolomite bodies were displaced by Pennsylvanian faults, whereas many dolomite bodies are related to the faults. Petrographically, nonstratal dolomite is finely to medium crystalline and has replaced recrystallized limestone. The dolomite has lower 87Sr/86Sr ratios (0.70772-0.70900; average 0.70834) and higher 18O values (+0.6 to -5.8; average -2.9), relative to associated (Lower Ordovician) limestone (87Sr/86Sr, 0.70878-0.70898, average 0.70887; 18O, -6.1 to -8.9, average -7.4). The petrographic and isotopic characteristics suggest that the nonstratal dolomite probably resulted from dolomitization of recrystallized limestone by post-Early Ordovician seawater. The relation of the nonstratal dolomite bodies to Pennsylvanian faults may indicate that the dolomite originated from multiple times of seawater circulatio through a paleokarst system. In the Arbuckle Anticline area, nonstratal dolomite is present as small (< 1 km2) irregular bodies that are related to Pennsylvanian faults and are associated with the margins of stratal Butterly dolomite. The nonstratal dolomite, medium to coarsely crystalline and brightly luminescent, is characterized by high (radiogenic) 87Sr/86Sr ratios (0.70951-0.70974), 18O) values (average -1.1), and Fe and Mn concentrations (average 1.32 mol % and 0.65 mol %, respectively), relative to all Arbuckle carbonates. Such compositions suggest that this type of dolomite probably originated from fluids that were derived from the adjacent basin(s) duri g late Paleozoic time.