Introduction to this special section: Rock physics

In simple terms, rock physics provides the much-needed link between measurable elastic properties of rocks and their intrinsic properties. This enables us to connect seismic data, well logs, and laboratory measurements to minerology, porosity, permeability, fluid saturations, and stress. Rock-physics relationships/models are used to understand seismic signatures in terms of reservoir properties that help in exploration risk mitigation. Traditionally, rock physics has played an irreplaceable role in amplitude variation with offset (AVO) modeling and inversion, 3D/4D close-the-loop studies, and seismic time-lapse analysis and interpretation. Today, rock-physics research and application have influenced a much wider space that spans digital rock physics, microseismic, and distributed acoustic sensing (DAS) data analysis. In this special section, we have included papers that cover much of these advanced methods, providing us with a better understanding of subsurface elastic and transport properties, thereby reducing bias and uncertainties in quantitative interpretation.

Geothermal Power Generation

Bulk power system based on fossil fuels becomes less reliable and stable in economic terms, technically more labor-consuming and harmful environmental impact. These problems have led many countries to find ways to supply the electricity from a green and sustainable energy source. The electricity derived from renewable energy sources such as hydro, solar, wind, biomass and geothermal refers to as green and sustainable energy. Geothermal energy is not only utilized for electric power generation, but it is also exploited to generate environmentally friendly heat energy. As of the end of 2018, geothermal global cumulative installed capacity exceeded 13 GW, generated an energy of about 630 peta joule (PJ). This chapter presents the geothermal energy resource in terms of the types of power plants, principle of the electricity generation and current world status of geothermal resource utilization. The issues such as advantages and disadvantages of geothermal energy economically and environmentally and means to overcome shortcomings are also considered. The main barriers for the development of geothermal industry include high resource and exploration risk, overall high development cost particularly drilling, and inadequate financing and grant support. The global averaged cost of electricity for the geothermal facility is nearly 0.072 USD/kWh as compared to 0.056 for onshore wind and 0.047 USD/kWh for hydropower. However, the technology is rather competitive to other renewables such as concentrating solar power (0.185 USD/kWh) and offshore wind (0.127 USD/kWh). Meanwhile, further research and development is critically needed to eliminate the non-condensable gases (NCGs) associated with the geothermal power generation.

Introduction to this special section: Basin exploration

Originally, explorers relied on hydrocarbon seeps and 2D seismic lines to define a basin's petroleum potential and to identify its best part. Due to dramatic improvements in seismic technology, today's explorers have access to a vast seismic tool kit, from basin-scale regional 3D seismic surveys to neural network techniques that directly derive reservoir properties from seismic volumes. However, in frontier or mature exploration, geoscientists are often limited by the amount of well data and rock properties information available, and they may deal with poor(er) seismic that makes seismic calibration difficult. Successful explorers will require a lot of knowledge-based creativity to derive meaningful geologic and reservoir properties models to reduce exploration risk. By embracing uncertainties, explorers develop “outside-the-box” thinking to help them chase the hydrocarbon molecules and unlock the prize. With their knowledge in rock physics, subsurface deformation, and wave propagation, geoscientists are deploying new methodologies and tools to fill the data gap and produce an improved description of the subsurface.

Rock physics and geomechanical application in the interpretation of rock property trends for overpressure detection

One of the complexities of geomechanical study is in the classification of rock’s properties and overpressured intervals—a knowledge which is not only essential for well safety and cost-effective drilling, but crucial in evaluating exploration risk factors and ensuring a successful hydraulic fracturing program. In this study, a more robust prediction of reservoir pressure regime is presented, where the geomechanical distributions of the rock give a distinct correlation. Three wells from the Niger Delta Basin were studied using empirical equations to estimate the elastic properties, wave velocities and the rock physics parameters for each well. From the results obtained, the velocities of compressional wave (Vp) and shear wave (Vs) decrease as porosity increases. Also, a linear correlation exists between Poisson’s ratio and Vp/Vs, where both variables showed distinct behavior and similar trend serving as useful tools for lithology identification. Another significant observation is the acoustic impedance of the materials which decreases with increasing porosity. Meanwhile, the depth plot of the impedance showed divergence and scattering away from the supposed linear trend. While inhomogeneity of the rock materials and disequilibrium compaction of sediments may account for this scattering, the variation of geomechanical distributions in this study revealed that pore pressure has a first order effect on the elastic strength of formations, also, under normal pore pressure conditions, acoustic impedance increases linearly with depth.

Petroleum Systems Analysis of The Tungkal PSC Area, South Sumatra – A Means of Adding New Life to A Mature Area

South Sumatra is considered a mature exploration area, with over 2500MMbbls of oil and 9.5TCF of gas produced. However a recent large gas discovery in the Kali Berau Dalam-2 well in this basin, highlights that significant new reserve additions can still be made in these areas by the re-evaluation of the regional petroleum systems, both by identification of new plays or extension of plays to unexplored areas. In many mature areas the exploration and concession award history often results in successively more focused exploration programmes in smaller areas. This can lead to an increased emphasis on reservoir and trap delineation without further evaluation of the regional petroleum systems and, in particular, the hydrocarbon charge component. The Tungkal PSC area is a good example of an area that has undergone a long exploration history involving numerous operators with successive focus on block scale petroleum geology at the expense of the more regional controls on hydrocarbon prospectivity. An improved understanding of hydrocarbon accumulation in the Tungkal PSC required both using regional petroleum systems analysis and hydrocarbon charge modelling. While the Tungkal PSC operators had acquired high quality seismic data and drilled a number of wells, these were mainly focused on improving production from the existing field (Mengoepeh). More recent exploration-driven work highlighted the need for a new look at the hydrocarbon charge history but it was clear that little work had been done in the past few year to better understand exploration risk. This paper summarises the methodology employed and the results obtained, from a study, carried out in 2014-15, to better understand hydrocarbon accumulation within the current Tungkal PSC area. It has involved integration of available well and seismic data from the current and historical PSC area with published regional paleogeographic models, regional surface geology and structure maps, together with a regional oil generation model. This approach has allowed a better understanding of the genesis of the discovered hydrocarbons and identification of areas for future exploration interest.

Open thermal convection dolomitization: an example from East Yunnan (China)

Dolostones are widely developed in the middle Permian rocks of East Yunnan, China, mainly in the shoal-facies Maokou Formation. The previously reported dolostone formation mechanisms cannot explain the distribution and geochemical characteristics of these dolostones, in particular their strontium, magnesium and oxygen isotope signatures. To help predict the distribution of dolostone reservoirs and reduce the exploration risk and cost, this study proposes a new model of dolomitization: open thermal convection dolomitization. In this new dolomitization model, Mg2+ in dolomitizing fluids originates mostly from seawater, with a minor component coming from deep hydrothermal fluids. Elevated heat flux (in this case due to the nearby Emei mantle plume) causes spatial temperature variations in the fluid along the circulation flow pathways, resulting in fast and pervasive dolomitization of limestone. The proposed model not only explains the characteristics and distribution of dolostones in the study area but also serves as a reference for predicting the distribution of dolostones in other areas subjected to thermal convection.