Paleokarst reservoir modelling - A concept-driven approach

A significant proportion of the world's hydrocarbon production comes from paleokarst reservoirs. Although these reservoirs boast some of the most productive wells in oil history, the recovery factor is relatively low (RFmean: 32%) compared to other carbonate reservoirs (RFmean: 37 - 51%). The low recovery could relate to current reservoir modelling approaches potentially yielding inaccurate resource estimates or early water-breakthrough. Conventional industry-standard reservoir modelling software suites do not have dedicated workflows or add-ins for handling the complex morphologies commonly associated with paleokarst. Current modelling approaches are often datadriven (conditioned on available seismic and well data) and employ adapted or modified versions of stochastic reservoir modelling workflows used for siliciclastic and carbonate reservoirs. However, many paleokarst features are below seismic resolution, and the representativity of individual well data is often challenging to assess. Consequently, data-driven models often fail to render the connectivity, geometry, and volume of karst features. Karst is the predecessor to paleokarst, and therefore a genetic approach employing existent information from recent karst systems may be a good starting point for generating analogues to paleokarst reservoirs. A concept-driven approach, in combination with current data-driven modelling approaches, may enable model rendering that more closely echoes actual paleokarst reservoir architectures. However, only a few conceptual modelling methods are publicly available and described in the literature. The drawbacks with the available methods are that they under-/overestimate the cave volumes, fail to provide realistic cave morphologies, and forecast clastic sediment infill, and do not differentiate between preserved and collapsed caverns. Consequently, post-collapse reservoir morphologies, volumes and facies distributions may be rendered inaccurately. This thesis aims to address the shortcomings of currently available conceptual methods and present a novel concept-driven workflow for paleokarst reservoir modelling. A novel methodology for geocellular rendering of karst systems is presented in this thesis. The method utilizes modern cave-survey data to generate dense, equally spaced point-clouds (infilling the cave periphery). These point clouds can be used to discretize the karst systems in a geocellular framework by geometrical modelling. The volumetric and geometric rendering of the method is compared with two pre-established methods and benchmarked against the cave survey. The results show that the new method offers improved volumetric and geometric geocellular rendering compared to the preestablished methods and are comparable to that of the cave survey. A pilot study using a well-known and pre-established geophysical method, electrical resistivity tomography (ERT), was carried out in the Maaras cave system in northern Greece to evaluate the large-scale volumetric significance and spatial distribution of clastic sediments infilling karst cavities. ERT proved to be a practical and useful method for differentiating mesoscale (>2.5 m2) stratigraphic heterogeneity. Resistivity contrasts allowed the identification of sedimentary thickness variations, interbedded breccias, and cave floor. Results showed that the siliciclastic sediment thickness varied from 25 m to >45 m, occupying a minimum of 69-95 % of the available accommodation space. Finally, a novel interactive tool for evaluating cavern stability and forward model collapse and infill processes was developed. The tool employs conventional cave survey data, field measurements and geomechanical data of the host rock to simulate potential post-collapse morphologies and generate spatial output data suitable for geocellular modelling. Collapse propagation, and eventually the volume affected by the collapse, is controlled by user-defined paleokarst facies proportions and associated average porosities following a “mass-balance-principle” (i.e., porosity is final and only redistributed over a larger volume). Three different collapse scenarios were modelled using the Agios Georgios cave system in northern Greece as an analogue. The results show that it is feasible to use cave surveys to simulate collapse and infill processes and estimate the final paleokarst reservoir architecture. The morphology, volume and relative facies-proportions rendered in the reservoir models are comparable to those calculated in the forward collapse modelling tool, indicating that the geocellular model echoes the simulation. The results also show that the vertical continuity and target volume of a reservoir increases significantly with increasing bedding dip. This suggests that improved forecasting of the final reservoir architecture may optimise well positioning, production planning and eventually improve recovery prediction.Doktorgradsavhandlin

High resolution digital 3D modelling of subsurface morphological structures of Koutouki Cave, Greece

Remote sensing techniques and laser scanning technology have given us the opportunity to study indoor environments, such as caves, with their complex and unique morphology. In the presented case study, we used a handheld laser scanner for acquiring points with projected coordinate information (X, Y, Z) covering the entire show cave of Koutouki; including its hidden passages and dark corners. The point cloud covers the floor, the walls, and the roof of the cave, as well as the stalactites, stalagmites and the connected columns that constitute the decoration of the cave. The absolute and exact placement of the point cloud within a geographic reference frame gives us the opportunity for three-dimensional measurements and detailed visualization of the subsurface structures. Using open - source software, we managed to make a quantification analysis of the terrain and generated morphological and geometric features of the speleothems. We identified 55 columns by using digital terrain analysis and processed them statistically in order to correlate them to the frame of the cave development. The parameters that derived are the contours, each column height, the speleothem geometry and volume, as well as the volume of the open space cavity. We argue that by the demonstrated methodology, it is possible to identify with high accuracy and detail: the geomorphological features of a cave, an estimate of the speleogenesis, and the ability to monitor the evolution of a karstic system.Key words: cave, laser scanner, 3D representation, speleothems, SLAM.  Visokoločljivostno digitalno 3D modeliranje podzemeljskih morfoloških struktur v jami Koutouki, Grčija Daljinsko zaznavne tehnologije in laserji nam omogočajo raziskovanje zaprtih prostorov, kot so jame z njihovo kompleksno in edinstveno morfologijo. Za pridobivanje različno dostopnih podatkov v turistični jami Koutouki smo v študiji uporabljali ročni laserski skener. Točkovni oblak pokriva tla, stene in strop jame ter stalaktite, stalagmite in stebre, ki prispevajo k okrasju jame. Natančna postavitev točkovnega oblaka v referenčnih geografskih okvirjih daje možnost tridimenzionalnih meritev in podrobne vizualizacije struktur v podzemlju. S podrobno analizo terena smo opredelili 55 stebrov in jih uvrstili v okvir razvoja jame. Pridobljeni so bili podatki o strukturiranosti, višini vsakega stebra, obliki in prostornini tamkajšnjih siginih tvorb ter prostornini prostega jamskega prostora. Z uporabljeno metodologijo je torej mogoče podrobno in z veliko natančnostjo opredeliti geomorfološke značilnosti jame, predvideti potek speleogeneze in spremljati razvoj kraškega sistema.Ključne besede: jama, laserski skener, 3D prikaz, sigine tvorbe, SLAM

Geoscience of the built environment: pollutants and materials surfaces

An overview of issues with environmental relevance that arise from the interaction between pollutants and surfaces of the built environment is presented in this paper. Two broad perspectives are considered: decay of materials and recording of pollution characteristics. In relation to the former, we consider the possible implications on human activities restrictions, materials and morphological options, consumption of resources and release of pollutants resulting from the alteration of materials, conservation and restoration procedures. In terms of pollution recording, the interest of the stony materials as passive monitors of pollution, the question of heterogeneous conditions on buildings and the interest of qualitative and quantitative studies are highlighted. The importance of longitudinal studies on new and cleaned surfaces is considered, both for the understanding of materials decay and for the assessment of pollution conditions. The use of tracers to record the characteristics of pollution sources, interaction with materials and pathways of pollutants is also discussed. Finally, some recommendations are presented, based on the issues discussed on this paper that might be relevant for environmental management programs, including environmental education.Fundação para a Ciência e Tecnologia (Portugal) (programa plurianual das unidades de investigação; PEst-OE/CTE/UI0697/2011)Fundação das Universidades PortuguesaMinisterio de Ciencia e Innovación - Acción Integrada PT2009-007

Towards understanding the Paleocean

A comparison is made between some of the framework used to discuss Paleoceanography and parallel situations in modern physical oceanography. A main inference is that too often the paleo literature aims to rationalize why a particular hypothesis remains appropriate, rather than undertaking to deliberately test that hypothesis.National Science Foundation (U.S.) (Grant No. OCE-0645936

The glacial-interglacial monsoon recorded by stalagmites from southwest Sulawesi, Indonesia

Climate variability in the tropics, and monsoons in particular, are critical to transporting moisture and heat around the earth. Deep atmospheric convection over the Indo-Pacific Warm Pool (IPWP) supplies heat and energy to two of the largest atmospheric circulation systems, the meridional Hadley Cell and zonal Indo-Pacific Walker Circulation. These convection centers are key components of the Australasian monsoon. Thus, the IPWP plays a fundamental role in global atmospheric circulation and hydrology. Paleoclimate records of IPWP climate have provided insight into the behavior of these major convective systems; however, significant differences between the reconstructions reveal a non-uniform response to various climate forcings. Recent modeling studies indicate that IPWP hydroclimate and convection are exceptionally sensitive to sea level and the exposure of the Sunda and Sahul shelves over glacial-interglacial (G-I) cycles, but the proxy evidence has not consistently supported this finding. Our study site, in southwest Sulawesi, is located within the IPWP and maintains a climatology dominated by the Australian-Indonesian summer monsoon (AISM). This site is an excellent location for tracking changes in IPWP climate variability and AISM strength through time. Stalagmite geochemical records have the potential to serve as precise monitors of past climatic and environmental change and can be absolute-dated to resolve terrestrial climate change on decadal to orbital timescales. We present eight new multi-proxy stalagmite records from the Maros karst in southwest Sulawesi, which collectively span 386–127 ky BP. The antiquity of the material inspired the design of a U-Th dating procedure using ultraviolet and transparency imaging to select the cleanest material, as well as geochemical assessment of U-Th data to construct robust and optimized chronologies. We investigated different age-depth modeling approaches and identified the Bayesian accumulation algorithm (BACON), coupled with tie-point optimization, as the ideal age-modeling technique for this work. Trace elements (Mg/Ca and Sr/Ca) in the Sulawesi stalagmites were assessed and found to derive primarily from the limestone host rock. Strong co-variation between Mg/Ca and Sr/Ca in slow-growing stalagmites demonstrates their sensitivity to prior calcite precipitation (PCP), and thus hydrological changes, whereas Mg/Ca and Sr/Ca in faster growing stalagmites appear to be influenced by additional factors. Using the hydrologically sensitive Mg/Ca data for two highly resolved (40–180 year-resolution), slow-growing stalagmites, we were able to test and confirm the reliability of stalagmite δ18O as a proxy for rainfall amount at our site. Coupled stalagmite δ18O and Mg/Ca records are particularly useful for investigating G-I changes in IPWP hydroclimate. The stalagmite δ18O record reveals coherent climatic features over G-I transitions. We focus on AISM variability during three periods of rapid global warming, glacial termination TIV (~340 ky BP) and both phases of TIII (TIII ~248 ky BP and TIIIa ~217 ky BP). Stalagmite δ18O and Mg/Ca data show rapid increases in rainfall during glacial terminations and wet interglacials. Glacial terminations are each characterized by an abrupt ~3‰ decrease in stalagmite δ18O that coincides with sea level rise, flooding of the Sunda and Sahul shelves, and strengthening of the AISM. Importantly, the good agreement of the δ18O and Mg/Ca records over G-I transitions demonstrates that southwest Sulawesi stalagmite δ18O is a reliable indicator of rainfall amount. The results show that, on G-I timescales, the strength of the AISM is most sensitive to the impact of changing sea level on the regional distribution of land and shallow-ocean. The new data, alongside an existing 40 ky record for southwest Sulawesi, provide eight transitional stages associated with G-I sea level change and reveal strong evidence for a shelf flooding/exposure threshold of approximately -65 ±9 m. Considered together, both glacial inceptions and glacial terminations imply a sea level threshold driving the AISM between two primary modes of intensity (‘interglacial’ and ‘glacial’). Precession-scale (23 kyr) monsoon variability associated with boreal autumn insolation at the equator is superimposed on these massive sea level driven shifts in AISM strength, indicating an underlying sensitivity of the AISM to the Walker Circulation’s influence on IPWP convection. When compared to stalagmite δ18O records for China, we find that AISM strengthening at glacial terminations consistently coincides with the final stages of weak monsoon intervals in the East Asian summer monsoon record, a key feature of global deglaciation. This observation, coupled with the inferred -65 m sea level threshold driving AISM strength, suggests that reinvigoration of the AISM, and thus IPWP convection, plays an important role in achieving and sustaining interglacial conditions. Although still in its infancy, this work provides critical insight into IPWP behavior over multiple G-I cycles, prompting new questions and inspiring collaboration with the paleoclimate modeling community