Quantifying occurrence of deformation bands in sandstone as a function of structural and petrophysical factors and their impact on reservoir quality: an example from outcrop analog of Productive Series (Pliocene), South Caspian Basin

Deformation bands (DB) are known to influence porosity and permeability in sandstones. This study aims to predict the occurrence of DB and to quantify their impact on reservoir properties based on field measurements in the steeply dipping limb of a kilometer-scale fold in Yasamal Valley, western South Caspian Basin. An integrated approach of characterizing bands and their effect on reservoir properties included measurements of natural gamma radioactivity and permeability using portable tools, along with bed dip and the count of DB across distinct facies. A set of core analyses was performed on outcrop plugs with and without bands to estimate the alteration of rock properties at the pore scale. Interpretation of outcrop gamma-ray data indicates the absence of bands in Balakhany sandstones containing shale volume greater than 18% for unconsolidated and 32% for calcite-rich facies. A high amount of calcite cement appears to increase the number of DB. A poor, positive trend between bed dip and DB concentration was identified. We show that net to gross, defined as the thickness fraction of sandstone bound by mudstones, is among the parameters controlling the occurrence of bands. Samples containing a single DB show a 33% and 3% decrease in permeability and porosity, respectively, relative to the host rock. We reveal a new set of lithological and petrophysical factors influencing DB occurrence. This study offers a direct tool that can be applied in subsurface reservoir analogs to predict the occurrence and concentration of DB and estimate their influence on rock properties.

The structure of the sedimentary complex of the Middle and South Caspian depressions (Azerbaijan sector)

The high degree of knowledge of the upper horizons of the sedimentary cover of the Middle and South Caspian depressions, given an insufficient increase in hydrocarbon reserves, leads to the need for a detailed approach to the search for oil and gas deposits in deep-seated sediments (over 6 km). During the geological interpretation of new highly informative seismic data, as well as data of deep drilling and petrological core studies, there were revealed obvious shortcomings in the concepts of the origin and evolution of the Middle and South Caspian depressions. These ideas misinterpret evolution, especially the South Caspian Basin, which is characterized by a number of unique features: very thick sedimentary cover (up to 22 km), extremely high sedimentation rate, low heat flow and reservoir temperatures, abnormally high pore and reservoir pressures, high clay content of the section, etc. The main purpose of the study was to elucidate the regional structure and features of the dissection of the sedimentary cover of the Middle and South Caspian depressions, the conditions of occurrence and distribution of facies and thicknesses of individual complexes of deposits. The paper analyzes the results of some previous studies of the geological structure of the Middle and South Caspian depressions based on the data of deep seismic sounding, seismological and gravimetric observations. We consider the main conclusions of these studies, about the geological structure of the sedimentary complex of the region’s, very outdated and subject to revision. The results of seismic stratigraphic analysis of seismic data allowed the authors to identify new data about the tectonic structure and express a completely different point of view regarding the structure of the sedimentary cover in the region. The work also touches on the issue associated with the tectonics of the region and the alleged subduction zone here.

Seismic Facies Analysis of Lower Productive Series of Gurgan-Deniz Field in South Caspian Basin with the Aid of Seismic Attributes

Drilled for the first time in 1946 and one of the oldest fields in the South Caspian Basin located in the western part of Apsheron sill, Gurgan-Deniz has been subject to redevelopment. A 3D seismic survey, conducted over the area for the first time, has been interpreted, analysing the lower Productive Series with regard to seismic facies and prospectivity. The facies analysis allows for better understanding of eustatic levels in the region and depositional environments of lower Productive Series in the area. A composite seismic attribute Sweetness and an RGB blend of Spectral Decomposition have been applied to the 3D volume, as well as to the interpreted stratigraphic surfaces. With the aid of the attributes and petrophysical well description, direct and indirect facies interpretation have been carried out. First, considering reflection parameters such as parallelism, continuity and hummockiness, as well as sedimentary features. Subsequently, reaching conclusions on depositional processes, environments, and geological evolution. Finally, analysing field prospectivity and migration pathways. Eight seismic facies have been identified by analysing stratigraphic horizons representing the tops of Kalin Suite (KaS), Pre-Kirmaky Sand Suite (PK) and Kirmaky Suite (KS). Facies have been interpreted as mass-flow deposits, amalgamated channel systems, channel and bar systems, sheetflow and floodplain deposits in a varying lacustrine-fluvial environment. KaS has been deposited following a sea- level drop and increased sediment inflow from Palaeo-Volga. The origin of the mass-flow facies is thought to be related to the increase of sedimentation speed, as well as tectonics decreasing the terrace stability. PK shows evidence of further sea-level drop and shows mainly fluvial depositional environment. Starting from KS, sea level has started to rise, once again showing mixed depositional environment. Attribute anomalies have been explored in the lower wing of the anticline structure in PK and KaS. A 3-way trap and possible migration pathways generate considerable risks.

State of Stress in Azeri-Chirag-Gunashli Anticline Structure, South Caspian Sea

The Azeri-Chirag-Gunashli (ACG) field, a tightly folded anticline structure, located offshore Azerbaijan in the south Caspian basin, is one of the most tectonically active regions of the world. Understanding the stress state across the ACG structure is a key to successful development of the field by optimizing well placement during drilling, completion, and depletion/injection phases. This article summarizes the results of studies undertaken on the state of the stress in ACG field. It encompasses the field-wide overview of stresses from a structural standpoint, the compilation of drilling and completion events, for instance, induced fracture during lost circulation events, and Formation Pressure Integrity Tests (FPIT) as well as analysis of wellbore breakout from variety of sources including borehole image data and caliper logs that were used to infer the magnitude and orientation of far-field stresses. Key outcomes of this study are the stress ratio distribution maps and stress orientation maps across the structure, based on the magnitude and orientations of stresses that were inferred from drilling events and wellbore breakouts. Results of this analysis show that magnitude and orientation of stresses vary across the structure both laterally and vertically. Minimum principal stress (Shmin) tends to increase from the crest toward the flanks. The maximum principal stress (SHmax) orientation is found to be predominantly sub-perpendicular to the strike of the anticline structure (60°-80° N), influenced by the reginal tectonic stresses. Moreover, stress rotation from sub-perpendicular to sub-parallel to the anticline is observed over some parts of the central and crestal areas, indicating that stresses are less compressional at the center and crest of the Azeri anticline. Local variability is possibly due to proximity to geological features such as mud volcanos, faults, and high deformation areas. The relative magnitude of stresses found in ACG, suggests a predominantly strike-slip faulting regime (where SHmax is the greatest of the three principal stresses) particularly, at the flanks and noses of the structure.

Multiscale nonlinear inversion of gravity data for depth-to-basement estimation via coupled stochastic-deterministic optimization

We have developed a multiscale approach for solving 2D and 3D nonlinear inverse problems of gravity data in estimating the basement topography. The inversion is carried out in two stages in which the long-wavelength features of the basement are first estimated from smoothed gravity data via a stochastic optimization algorithm. The solution of this stage is used as the starting model for a deterministic optimization algorithm to reconstruct the short-wavelength features from the full-spectrum gravity data. The forward problem is capable of handling lateral and vertical variations in the density of sediments. Two cases are considered regarding prior knowledge about the density: (1) The density contrast between sediments at the surface and the underlying basement and its vertical variations are a priori known, and (2) only the density contrast at the surface is known with its vertical gradient to be recovered in the inversion. In the former case, the unknowns of the problem are the depths, whereas in the latter case, they are the depths and density gradients defined individually for each prism. Therefore, the inverse problem is ill-posed and has many local minima. The stochastic optimization algorithm uses a random initial model and estimates a coarse model of the basement topography. By repeating the stochastic inversion, an ensemble of solutions is formed defining an equivalent domain in the model space supposed to be within the neighborhood of the global minimum of which several starting solutions are extracted for the secondary deterministic inversion. The presented methodology has been tested successfully in converging to the global minima in 2D and 3D cases with 50 and 2352 total number of prisms, respectively. Finally, the inversion algorithm is used to calculate the thickness of the sediments in the South Caspian Basin using the EIGEN-6c4 global gravity model.

Effect estimation of variations of petrophysical and lithological-facial characteristics on the quality of reservoir rocks

The paper presents the results of researches on the specification of clay components, as well as the zones of intra- and interstratal fluid flows in the formation processes of oil-gas fields. The investigations have been carried out in the context of Sangachal-Duvanny-Khara-Zira (SDKhZ) field, located in Baku archipelago of South Caspian basin (SCB). It was defined that the presence of clayey chlidolite in the reservoirs are observed more effective in the values of permeability rates. The zones of intra- and interstrata of fluid flows in the reservoirs were specified based on the lithological model of productive horizons. Therefore, the presence of closed migration-flow (drainage) system both within productive horizons and in the scale of reviewed field was supposed. In support of these assumptions, 2D and 3D variation models of flow zones indicator values for intrastrata levels of productive horizons and for the section fragment of SDKhZ field have been developed.