Compaction-induced Subsidence Research Articles

Syndepositional fractures and architecture of the lastoni di formin carbonate platform: Insights from virtual outcrop models and field studies

The recent rapid improvement of Unmanned Aerial Vehicles, together with advances in photogrammetry and Structure from Motion techniques, have enhanced the role of Digital Outcrop Models in many field of geology, due to the possibility to obtain quantitative information from large and inaccessible areas. In this study we integrated Digital Outcrop Modeling techniques and field survey to investigate the architecture of the Middle Triassic platform of Lastoni di Formin. (Italian Dolomites). The research highlighted the presence of two superimposed carbonate bodies. The lower unit (Cassian I) is dominated by low-angle clinoforms dipping north-northeastward and prograding over the basinal San Cassiano Fm. The upper unit (Cassian II) is characterized by a thick sequence of peritidal cycles connected northward to another generation of clinoforms. The inner platform beds of the upper unit display a lateral thickening that is particularly evident near the shelf break, and that has been interpreted as due to the increased subsidence and the consequent down-to-basin tilting of the outermost part of the platform. Moreover, the structural analysis performed on the Digital Outcrop Models and supported by field observations, highlighted the presence of an early generation of faults and joints that indicate an early gravitational deformation of the buildup, possibly caused by the platform progradation and compaction-induced subsidence of the San Cassiano basinal deposits. These WNW-ESE synsedimentary structures are formed by normal faults and extensional joints that are oriented nearly perpendicular to the direction of progradation of the carbonate platform.

Process Controls of the Live Root Zone and Carbon Sequestration Capacity of the Sundarbans Mangrove Forest, Bangladesh

The conservation of coastal wetland ecosystems, like mangrove forests and salt marshes, represents a critical strategy for mitigating atmospheric emissions and climate change in the 21st century. Yet the existence of these environments is threatened by human-induced disturbances, namely deforestation and accelerated sea-level rise. Coastal systems maintain surface elevation in response to sea-level rise through a combination of physical and biological processes both above and below the ground surface. The quantification and relative contribution of belowground process controls (e.g., seasonal water content, organic matter decomposition) on surface elevation change is largely unexplored but crucial for informing coastal ecosystem sustainability. To address this knowledge deficit, we integrated measurements of surface elevation change of the live root zone (0.5 to 1 m depth) with geotechnical data from co-located sediment cores in the Sundarbans mangrove forest (SMF) of southwest Bangladesh. Core data reveal that the primary belowground controls on surface elevation change include seasonal fluctuations in pore-water content and the relative abundance of fine-grained sediments capable of volumetric expansion and contraction, supporting an elevation gain of ~2.42 ± 0.26 cm year−1. In contrast to many mangrove environments, the soils of the SMF contain little organic matter and are dominantly composed (>90%) of inorganic clastic sediments. The mineral-rich soil texture likely leads to less compaction-induced subsidence as compared to organic-rich substrates and facilitates surface equilibrium in response to sea level rise. Despite a relatively high soil bulk density, soil carbon (C) density of the SMF is very low owing to the dearth of preserved organic content. However, rates of C accumulation are balanced out by locally high accretion rates, rendering the SMF a greater sink of terrestrial C than the worldwide mangrove average. The findings of this study demonstrate that C accumulation in the SMF, and possibly other alluvial mangrove forests, is highly dependent on the continued delivery of sediment to the mangrove platform and associated settings.

Process Controls of the Live Root Zone and Carbon Sequestration Capacity of the Sundarbans Mangrove Forest, Bangladesh

The conservation of coastal wetland ecosystems, like mangrove forests and salt marshes, represents a critical strategy for mitigating atmospheric emissions and climate change in the 21st century. Yet the existence of these environments is threatened by human-induced disturbances, namely deforestation and accelerated sea-level rise. Coastal systems maintain surface elevation in response to sea-level rise through a combination of physical and biological processes both above and below the ground surface. The quantification and relative contribution of belowground process controls (e.g., seasonal water content, organic matter decomposition) on surface elevation change is largely unexplored but crucial for informing coastal ecosystem sustainability. To address this knowledge deficit, we integrated measurements of surface elevation change of the live root zone (0.5 to 1 m depth) with geotechnical data from co-located sediment cores in the Sundarbans mangrove forest (SMF) of southwest Bangladesh. Core data reveal that the primary belowground controls on surface elevation change include seasonal fluctuations in pore-water content and the relative abundance of fine-grained sediments capable of volumetric expansion and contraction, supporting an elevation gain of ~2.42 ± 0.26 cm yr−1. In contrast to many mangrove environments, the soils of the SMF contain little organic matter and are dominantly composed (>90%) of inorganic clastic sediments. The mineral-rich soil texture likely leads to less compaction-induced subsidence as compared to organic-rich substrates and facilitates surface equilibrium in response to sea level rise. Despite a relatively high soil bulk density, soil carbon (C) density of the SMF is very low owing to the dearth of preserved organic content. However, rates of C accumulation are balanced out by locally high accretion rates, rendering the SMF a greater sink of terrestrial C than the worldwide mangrove average. The findings of this study demonstrate that C accumulation in the SMF, and possibly other alluvial mangrove forests, is highly dependent on the continued delivery of sediment to the mangrove platform and associated settings.

Process Controls of the Live Root Zone and Carbon Sequestration Capacity of the Sundarbans Mangrove Forest, Bangladesh

The conservation of coastal wetland ecosystems, like mangrove forests and salt marshes, represents a critical strategy for mitigating atmospheric emissions and climate change in the 21st century. Yet the existence of these environments is threatened by human-induced disturbances, namely deforestation and accelerated sea-level rise. Coastal systems maintain surface elevation in response to sea-level rise through a combination of physical and biological processes both above and below the ground surface. The quantification and relative contribution of belowground process controls (e.g., seasonal water content, organic matter decomposition) on surface elevation change is largely unexplored but crucial for informing coastal ecosystem sustainability. To address this knowledge deficit, we integrated measurements of surface elevation change of the live root zone (0.5 to 1 m depth) with geotechnical data from co-located sediment cores in the Sundarbans mangrove forest (SMF) of southwest Bangladesh. Core data reveal that the primary belowground controls on surface elevation change include seasonal fluctuations in pore-water content and the relative abundance of fine-grained sediments capable of volumetric expansion and contraction. In contrast to many mangrove environments, the soils of the SMF contain little organic matter and are dominantly composed (>90%) of inorganic clastic sediments. The mineral-rich soil texture likely leads to less compaction-induced subsidence as compared to organic-rich substrates and facilitates surface equilibrium in response to sea level rise. Despite a relatively high soil bulk density, soil carbon (C) density of the SMF is very low owing to the dearth of preserved organic content. However, rates of C accumulation are balanced out by locally high accretion rates, rendering the SMF a greater sink of terrestrial C than the worldwide mangrove average. The findings of this study demonstrate that C accumulation in the SMF, and possibly other alluvial mangrove forests, is highly dependent on the continued delivery of sediment to the mangrove platform and associated settings.

Estimation of sediment compaction and its relationship with river channel distributions in the Yellow River delta, China

The geomechanical properties of the stratigraphic sequences strongly influence the land surface subsidence, as the latter can be caused by the sediment compaction. Depositional units of transitional environments such as coastal plain and deltas are characterized by a high lithological variability due to the history and complexity of the sedimentary processes. Therefore, detailed modeling of the stratigraphic architecture is necessary to better understand the land surface subsidence process and accurately estimate its rates. The frequent avulsion and rearrangement of the channel network in the modern Yellow River delta since 1855 has led to the rapid accumulation and deposition of large amounts of sediment in the delta plain. The compaction of the substratum sediment due to the new sediment load and the consolidation of younger deposits caused land surface subsidence. We used 108 sediment core data covering the delta and obtained detailed stratigraphic structural information. A detailed distribution of clay and soft clayey soils, which are the primary contributors to land subsidence, was identified. Furthermore, the compaction rates and the consolidation at time of the subaerial area and neritic area for the entire modern Yellow River delta were estimated. We also analyzed the characteristics of present land subsidence by combining a ground movement dataset obtained by high-resolution topographic surveys with Synthetic Aperture Radar (SAR) Interferometry (InSAR) of satellite images. The results show that during the early stages of the development of the active deltaic lobe, the compaction of neritic sediments was the primary contributor to initial subsidence. Furthermore, combined with the relevant sedimentary age and environment determined by previous studies, we found that the spatial variation in compaction-induced subsidence in the subaerial deltaic area over time is closely related to the historical distributions of the river channels in the modern Yellow River delta. Our findings, providing a better characterization of the spatial and time heterogeneity of the land surface subsidence of both subaerial and submarine areas, significantly improved the previous knowledge on land subsidence of the Yellow River delta. Our results also demonstrate that the subaerial delta becomes increasingly stable over time with the gradual decrease in the average subsidence rate of each deltaic lobe.

Does compaction-induced subsidence control accommodation space at the top of prograding carbonate platforms? Constraints from the numerical modelling of the Triassic Esino Limestone (Southern Alps, Italy)

The demise of the high-relief, steep-slope, prograding Ladinian-Early Carnian carbonate platforms of the Esino Limestone (Central Southern Alps of Italy) is marked by subaerial exposure of the platform top associated with different erosional (mainly karst-related), depositional and diagenetic processes (Calcare Rosso). The exposure-related deposits consist of three major facies associations: 1) residual soils with thin lenses of conglomerates with black pebbles, and, locally, weathered vulcanites; 2) chaotic breccia lenses irregularly distributed in the uppermost part of the Esino Limestone carbonate platform, interpreted as collapse breccias in karstic setting: 3) inter-supratidal carbonate cycles with dissolution and development of paleosols and tepee structures.Facies distribution follows the sub-environments of the underlying Esino Limestone. Facies 1 and 2 typically characterize the core of the platform, covering the underlying inner platform facies. Facies 3 instead develops toward the edge of the platform, above reef-upper slope facies of the prograding facies of the Esino Limestone. The thickness of facies 3 decreases toward the core of the platform. Facies distribution reflects differences in the accommodation space and sedimentary processes from the rim (highest accommodation, favouring the deposition of peritidal-supratidal carbonates) to the core (reduced accommodation, causing pedogenesis and karstification) of the carbonate system.The observed thickness changes may be controlled by different factors: 1) syndepositional tectonics, 2) subsidence induced by magmatic activity or 3) differential subsidence controlled by the stratigraphic architecture of the Esino Limestone platform and adjoining basins. As evidence of tectonics was not observed and the presence of volcanic bodies is only documented tens of km away from the study area, the scenario involving the creation of accommodation space by compaction of the basinal sediments (resedimented, fine-grained calciturbidites) during the progradation of the carbonate platform is here investigated. Numerical modelling was performed to verify the compatibility of compaction-induced subsidence with the observed depositional architecture. The models were built to simulate the architectural evolution of the platform by progressively adding layers from deepest to shallowest, while compacting the underlying sediments, in order to evaluate compaction-induced subsidence (and accommodation space for the Calcare Rosso) after the deposition of the youngest platform strata. Modelling results allow us to conclude that the wedge geometry of the Calcare Rosso, deposited on top of the extinct Esino carbonate platform, can be explained by subsidence controlled by compaction of the basinal sediments present below the early-cemented, fast prograding platform slope deposits.

Deriving thrust fault slip rates from geological modeling: Examples from the Marche coastal and offshore contraction belt, Northern Apennines, Italy

We present a reconstruction of the central Marche thrust system in the central-northern Adriatic domain aimed at constraining the geometry of the active faults deemed to be potential sources of moderate to large earthquakes in this region and at evaluating their long-term slip rates. This system of contractional structures is associated with fault-propagation folds outcropping along the coast or buried in the offshore that have been active at least since about 3 Myr. The ongoing deformation of the coastal and offshore Marche thrust system is associated with moderate historical and instrumental seismicity and recorded in sedimentary and geomorphic features. In this study, we use subsurface data coming from both published and original sources. These comprise cross-sections, seismic lines, subsurface maps and borehole data to constrain geometrically coherent local 3D geological models, with particular focus on the Pliocene and Pleistocene units. Two sections crossing five main faults and correlative anticlines are extracted to calculate slip rates on the driving thrust faults. Our slip rate calculation procedure includes a) the assessment of the onset time which is based on the sedimentary and structural architecture, b) the decompaction of clastic units where necessary, and c) the restoration of the slip on the fault planes. The assessment of the differential compaction history of clastic rocks eliminates the effects of compaction-induced subsidence which determine unwanted overestimation of slip rates. To restore the displacement along the analyzed structures, we use two different methods on the basis of the deformation style: the fault parallel flow algorithm for faulted horizons and the trishear algorithm for fault-propagation folds. The time of fault onset ranges between 5.3 and 2.2 Myr; overall the average slip rates of the various thrusts are in the range of 0.26–1.35 mm/yr.

Factors Controlling Peat Compaction in Alluvial Floodplains: A Case Study in the Cold-Temperate Cumberland Marshes, Canada

Abstract Subsidence due to peat compaction may have important implications in alluvial floodplains, because it leads to wetland loss, land inundation, and damage to buildings and infrastructure. Furthermore, it potentially influences spatial and temporal river sedimentation patterns, and hence the evolution of alluvial floodplains. As the vast majority of peatlands occur in cold temperate regions, alluvial plains situated in such areas are most susceptible to these implications. To determine which locations within alluvial floodplains are most vulnerable to high amounts of compaction-induced subsidence, it is necessary to quantify compaction and to identify which factors influence the amount and rate of peat compaction. For this, we carried out field work in the Cumberland Marshes (east-central Saskatchewan, Canada) guided by the following two questions: (1) how much peat compaction has occurred over the past decades to centuries, and (2) which factors control the amount and rate of peat compaction at the same timescale. To address these questions, we used methods involving construction of stratigraphic cross sections, organic-matter content and bulk-density measurements, and radiocarbon dating. A new sampling device was devised to sample uncompacted peat in the field. Results show that peat layers have compacted up to 43% within a few centuries, with compaction rates of up to 6.08 mm/yr. The dominant factors influencing peat compaction are: (1) organic-matter content, (2) stress imposed on a peat layer, and, to a lesser extent, (3) plant species composition. In an alluvial setting, crevasse splays and natural levees are sites that are most susceptible to high amounts of peat compaction at short timescales (100–102 years). Sheet-like splay deposits initially compact underlying peat uniformly, whereas differential compaction commonly occurs beneath natural levees due to lateral variations in sediment thickness. Subsidence due to peat compaction creates additional accommodation space and hence locally enhances floodplain sedimentation rates.

Unraveling reservoir compaction parameters through the inversion of surface subsidence observations

In an attempt to derive more information on the parameters driving compaction, this paper explores the feasibility of a method utilizing data on compaction-induced subsidence. We commence by using a Bayesian inversion scheme to infer the reservoir compaction from subsidence observations. The method’s strength is that it incorporates all the spatial and temporal correlations imposed by the geology and reservoir data. Subsequently, the contributions of the driving parameters are unravelled. We apply the approach to a synthetic model of an upscaled gas field in the northern Netherlands. We find that the inversion procedure leads to coupling between the driving parameters, as it does not discriminate between the individual contributions to the compaction. The provisional assessment of the parameter values shows that, in order to identify adequate estimate ranges for the driving parameters, a proper parameter estimation procedure (Markov Chain Monte Carlo, data assimilation) is necessary.

Differential compaction as a control on depositional architectures across the Maiella carbonate platform margin (central Apennines, Italy)

The role of differential compaction as a control in the creation of accommodation and on compaction-modified depositional features and stratal geometries across the Maiella platform margin, has been investigated through a combined analysis of seismic scale outcrops, porosity evaluation and modeling. Geologic evolution and large exposures make the platform margin of the Maiella an ideal place to investigate the effects of differential compaction. A high-relief cemented carbonate platform, a deep basin filled with highly compactable deposits, and a prograding grain-rich succession sealing morphologic differences across the platform margin, represent suitable features for promoting differential compaction. Stratal relationships across the platform margin exhibit evidence of differential compaction-induced effects, such as basinward divergence and thickening of strata, updip pinch-out of wedge-shaped stratal packages, and an anticline hinge. Porosity analysis and modeling indicate that, through progressive loading, mechanical and chemical processes act in concert to destroy most of the depositional porosity. Mechanical compaction appears to have played the greatest part in the total budget of compaction. However, chemical compaction seems to have played a prominent role in the formation of geometrically consistent depositional profiles during progradation. Due to differential compaction across the platform margin a compaction hinge formed concomitantly with the beginning of progradation, producing a basin-facing monocline characterized by the progressive steepening of basinward stratal dips. The resulting compaction-induced stratal deformation, together with sea level changes, controls the distribution, and depositional timing of wedge-shaped stratal packages during late Cretaceous and Paleocene and the distribution of coral–algal reef buildups, during the late Eocene–early Oligocene. The development of the compaction hinge usually follows the progressive increase of loading, but a decrease in compaction dissipation may be caused by overburdening of compactable deposits. This will cause a delayed compaction-induced subsidence, whose effects will be produced after deposition, i.e. during loading interruption. This mechanism is thought to have an important role in the timing of compaction-induced subsidence throughout the end of Cretaceous and the early Tertiary, and it is considered, together with sea level, as the main controlling process of the occurrence and distribution of downslope sediments, during a long-lasting period of platform emersion.

Quantitative Meso-/Cenozoic development of the eastern Central Atlantic continental shelf, western High Atlas, Morocco

The well exposed Late Paleozoic to Cenozoic succession in the western High Atlas (Morocco) documents the early rift to mature drift development of the eastern Central Atlantic continental shelf basin. Vertical sections, depositional geometries and unconformities have been used to reconstruct the basin architecture prior to Atlasian inversion. Two-dimensional reverse basin modeling has been performed to quantitatively analyze the development of the continental shelf between the latest Paleozoic to Early Cenozoic. Basin evolution stages include (i) early rift, Late Permian to Anisian; (ii) rift climax, Ladinian to Carnian; (iii) sag, Norian to Early Pliensbachian; (iv) early drift, Late Pliensbachian to Tithonian; (v) mature drift, Berriasian to Cenomanian; (vi) mature drift with initial Atlasian deformation, Turonian to Late Eocene; (vii) Atlasian deformation; Late Eocene to Early Miocene; (viii) Atlasian uplift and basin inversion, Early Miocene to Recent. The Late Permian to Late Cretaceous basin development comprises eight subsidence trends of 10–35 Myr duration. Trends were initiated by changes in thermo-tectonic subsidence, which in turn triggered positive and negative feedback processes between sediment flux, flexural and compaction-induced subsidence. Plate-tectonic reconfigurations in the Atlantic domain controlled the thermo-tectonic subsidence history and the basin development of the Agadir segment of the northwest African passive continental margin: (1) major shifts in the sea-floor spreading axis; (2) significant decreases in sea-floor spreading rates; (3) the stepwise migration of crustal separation and sea-floor spreading in and beyond the Central Atlantic; (4) African–Eurasian relative plate motions and convergence rates. During the Early Pliensbachian/Toarcian to Cenomanian the first three plate-tectonic reconfigurations triggered changes in ridge-push forces and modulated the extensional stress field of Central Atlantic plate drifting. Since the Turonian, African–Eurasian relative plate motions and convergence rates represented the dominant control on the thermo-tectonic subsidence history in the Agadir Basin. Major variations in sediment flux and total subsidence characterize the development of the northwest African passive continental margin. The explanation of typical stratigraphic sequences as caused predominantly by sea-level fluctuations, and rough assumptions on sediment input/production and subsidence, is not necessarily applicable to passive continental margins. The methodology applied in this study, including the newly developed tool of ‘Compositional Accommodation Analysis’, allows to develop more rigorous genetic models for the development of continental shelf basins.

Nearfield vertical displacement in the creeping segment of the San Andreas fault, central California, 1975 to 1994

Nearfield precise spirit leveling reveals progressive, aseismic (?) height changes in the creeping segment of the San Andreas fault between Gold Hill in Cholame Valley and San Juan Bautista over time periods from 8 to 18 years. Height changes are greatest at each end of the creeping segment, whereas the rate of horizontal creep is greatest in the center of the creeping segment. Both vertical and horizontal displacements take place at the surficial trace of the fault, whereas surficial downwarping is evident as much as 500 m from the mapped fault trace. The large height changes at San Juan Bautista, Parkfield, and Gold Hill may be related to compaction-induced subsidence where groundwater is pumped from unmonitored irrigation wells on one side of the fault. At San Juan Bautista, however, the water table has remained nearly constant according to local farmers, even though the height change there is the greatest among the six sites. Therefore, it is concluded here that the vertical displacements are tectonic and result from local deviations of the direction of fault slip relative to the far-field slip vector, or alternatively to deceleration of horizontal creep and dissipation of horizontal strain into the transition zones between the end of the creeping fault segment and the adjacent locked segments.

System Tract History Analysis: Key to Understanding Deposition in Cored Reservoir Analog Units on the Shelf

ABSTRACT An analytical procedure: system tract history analysis, has been developed to integrate high-resolution bio-chronostratigraphy, sediment accumulation, compaction-induced subsidence, and late Quaternary sea level records into a detailed accommodation space framework for characterizing system tract deposition. The technique, a modification of geo-history analysis has been profitably applied to section cored on the mid-Louisiana shelf in ca. 70 m of water. Analysis of the cored section places a wave-based induced erosional fourth order sequence boundary at ca. 75 kyBP, within the isotope stage 4 sea level low-stand. It further indicates that a channel-cut, forced regression surface, within a reservoir analog low-stand delta unit, marks a ca. +30 m terrace that was extant during a portion of the intense isotope stage 2 sea level low-stand which culminated ca. 18 kyBP. Bounding transgressive and highstand system tracts are consistent with deposition in middle neritic depths (ca. 60 to 90 m). End_of_Record - Last_Page 514-------

Eustatic sea level fluctuations induced by polar wander

THE observation of apparently simultaneous episodes of deposition or non-deposition of marine sediments in different parts of the world has led to the proposal that at least some sea level rises and falls must be global, or eustatic, in character1–3. Here we show that polar wander of a viscoelastic stratified Earth can induce global sea level fluctuations comparable to the short-term component in the eustatic sea-level curves. The sign of these fluctuations, which are very sensitive to the rheological stratification, depends on the geographical location of the observation point; rises and falls in sea level can thus be coeval in different parts of the world. This finding is in distinct contrast to the main assumption underlying the reconstruction of eustatic curves, namely that global sea-level events produce the same depositional sequence everywhere. This apparent contradiction is due to the poor time resolution of the stratigraphie records in the distant past, which is comparable to the timescale of polar motion4–6, and to non-uniform data coverage. We propose that polar wander should be added to the list of geophysical mechanisms (the others are glacial instabilities, plate tectonic mechanisms, sea-floor spreading, and thermal and compaction-induced subsidence) that can control the third-order cycles in sea level.

Tectonic evolution of the Northumberland Basin; the effects of renewed extension upon an inverted extensional basin

The Northumberland sedimentary basin has a history of repeated extensional and contractional tectonic phases and a study of the deformation interference pattern and sedimentary record allows the identification of the tectonic state through time. Initial rifting is represented by a thick Dinantian sedimentary fill, but Carboniferous extension is thought to have ceased by the early Namurian. In order to explain the development of certain facies patterns during the Westphalian thermal subsidence phase of the evolution of the Northumberland Basin, some syn-depositional extensional fault movements are required. At other times in the Westphalian, compaction-induced subsidence was the dominant control on facies distribution. A single phase of E–W to SE–NW Variscan compression commenced during or after late Westphalian times and ceased prior to the intrusion of the Whin sills and dykes. Subsequent Permo-Mesozoic extension, with a right-lateral shear component, re-activated pre-existing normal fault lines. These later structures may be interpreted as being listric in form, shallowing onto decollement surfaces within the Carboniferous basin fill and overlie the buried faulted margin at the southern side of the basin.

Occurrence of a Pseudophragmina (Proporocyclina) Zaragosensis Bank in Upper Wilcox (Lower Eocene), Pointe Coupee Parish, Louisiana: ABSTRACT

ABSTRACT Rock-forming buildups of larger foraminifera, while not unknown in Louisiana, are rare, and are less common in the Gulf Coast than in Eocene strata from the Pyrenean and Caribbean regions. In strata from the Upper Wilcox Group (lower Eocene) in Pointe Coupee Parish, Louisiana, we have identified a thin, very rich faunal deposit, or bank, made up almost exclusively of the tests of the larger foraminifera species, Pseudophragmina (Proporocyclina) zaragosensis. This is the first report of P. (P.) zaragosensis from the Wilcox Group of Louisiana. The bank fauna colonized the sandy sea floor on the continental shelf, flourished briefly, and then gradually declined, probably in response to increasing water depth, caused either by compaction-induced subsidence, a local or regional rise in relative sea level or both. This bank indicates that during late Wilcox time, warm shallow continental shelf conditions prevailed in this region.

Basin Evaluation Using Burial History Calculations: an Overview

Burial history calculations available to the petroleum geologist include plots of burial history and geohistory, porosity, and conductivity as a function of depth and lithology, as well as the results obtained from algorithms to handle unconformities and backstripped crustal subsidence, calculations that predict the breakdown of kerogen to hydrocarbons in terms of time and temperature, and methods for determining heat flow from geologic models and vitrinite reflectance. Data inputs required for these calculations include depths of formation tops, ages of formation tops, lithologies of formations, depths of water at deposition of the different formations, porosities of the formations as a function of depth, bottom-hole temperatures and/or formation temperatures, and any ke ogen content and vitrinite reflectance values as functions of depth. Sources of many of these data may be well logs, well reports, geologic papers, and/or seismic sections. The paper considers how parameters can be extracted from burial-history calculations to construct maps that can be compared to the current locations of oil and gas fields, and so used to locate and rank prospective acreage. These parameters include the potential source rock maturity of a formation plotted as a function of time and depth, the thermal history of any hydrocarbons associated with the potential source rock, the rate of sediment accumulation of a formation as a function of time, and subsidence rate of a formation as a function of time. Subsidence rate includes total subsidence, compaction-induced subsidence, isostatic response to sediment load, and backstripped crustal subsidence (total subsidence minus the effects of compaction and isostatic response).

The relationship between formation structure and thickness in the Permo-Triassic succession of the Southern Coalfield, Sydney Basin, New South Wales, Australia

Using trend-surface analysis, the patterns of formation structure and thickness variation in the Permo-Triassic succession of the Southern Coalfield, Sydney Basin, are resolved into geologically meaningful large- and small-scale features. The structural trends correspond to the regional syncline of the southern Sydney Basin and the negative and positive residual domains to smaller-scale structures including the South Bulli and Douglas Park Synclines and the Bulli and Kemira Anticlines. The results of trend-surface analysis and simple linear regression reveal an inverse relationship between structural elevation (relative to sea level) and formation thickness on both a regional and a local scale. This relationship is strongest for the Bulli Coal and the Bald Hill Claystone. In the case of the Bulli Coal, the present-day structural trends and formation thickness trends show a strong inverse correlation and the residuals show a moderate inverse correlation; the correlations between the inferred Middle Triassic structure of the Bulli Coal and formation thickness are even stronger than those involving the present-day structure. These results indicate that structures which were active during the Permo-Triassic and which influenced sedimentation have been preserved in the present-day structure. The persistence of the patterns of structure and thickness variation upward through the Permo-Triassic sequence suggests that sedimentation was controlled by contemporaneous basement subsidence, rather than by short-lived compaction-induced subsidence patterns.