Seismic interpretation is a key step in exploration, appraisal or field evaluation and optimized development. This could be re-interpretation of (re-processed) existing seismic data (usually in field evaluation and development planning, or interpretation of newly acquired data.
Seismic data acquisition is normally executed by Contracted Companies. Essential hereby is that the speed; quality and cost of acquisition and processing operations are as per contract. Some Clients have departments watching these operations closely. During data acquisition we deliver experienced QA / QC engineers. In additon, OPS OES Thailand provides experienced geophysicists who can take up this role completely or represent Client herein.
Supervision Seismic Data Acquisition and Processing
Our experienced geophysicists are fully capable of setting up contracts with geophysical survey companies to plan and complete new seismic surveys in a timely and cost-effective fashion. They will be closely monitoring the progress of the seismic data acquisition and data processing, and will ensure the execution and the quality of the works is according contract specifications. They have excellent understanding of seismic processing workflows and algorithms.
Interpretation of 2D, 3D & 4D Seismic Data
Seismic interpretation represents a key step in any exploration, appraisal, or field evaluation study. In many instances, it is the very start of exploration. Using software platforms such as Petrel, IHS Kingdom, GeoGraphix or other industry standard interpretation and mapping platforms, geophysicists provided by OPS OES undertake a range of geophysical studies and interpretations to solve the structural-stratigraphic framework in structurally complex regions or basins or at prospective horizons for exploration of hydrocarbons. Seismic interpretation is also a key element in total field reviews. In the latter, seismic 4D interpretation can help the client to detect changes in travel time as a result of fluid production, or injection in reservoir-drive reservoirs, not just in the reservoir body, but also in the overburden due to compaction, which can be monitored and interpreted using time-lapse seismic data, as illustrated below.
4D analysis and how the reservoir changes over time; reservoir production causes deformation in reservoir and surrounding rock (source Schlumberger).
Time-Depth Conversion, Well Tie, Depth Structure Mapping
Our geophysicists will handle the well seismic calibration, time-to-depth conversion and thus ensure a reliable depth structure mapping. In addition, they will also be responsible for managing the geophysical database.
Seismic Attribute Analysis
In a seismic survey, seismic amplitude is a measure of the contrast in properties between two layers. If the data are converted to relative impedance then seismic data observe the relative change of the rock property impedance (= hardness) as we move from one layer to the next.
As such, seismic data are analyzed in order to enhance information that might be subtler in a traditional seismic image, leading to a better geological or geophysical interpretation of the data. Examples of seismic attributes can include measured time, amplitude, frequency and attenuation, AVO.
Picking top and base zero crossing to define the thickness and amplitude of hard (turbidite) sand from relative impedance seismic inversion data (source: GEO EXPRO, Vol. 12, No.5 – 2015).
Net sand map (m) of a turbidite fan , resulting from multiplying N/G map by the time thickness map and also multiplying by the sand velocity (source: GEO EXPRO, Vol. 12, No.5 – 2015).
Individual seismic attributes give us indications of many subtle reservoir properties. However combining multiple seismic attributes will give the geophysicist a much better and more reliable prediction and ability to solve the non-linear relationship between the seismic attributes and reservoir properties.
The improved geologic interpretation and prediction (e.g. of production indicators) assist in the identification of preferential drilling locations and completion practices. Interaction between the geophysicist, geologist and reservoir- and drilling engineers is therefore of utmost importance.
A seismic attribute that can indicate the presence or absence of hydrocarbons is known as a direct hydrocarbon indicator (DHI). DHI’s are visible as bright spots on seismic data.
Seismic Facies Analysis
OPS OES provides geophysicists, who are experienced in the mapping of three-dimensional seismic units composed of groups of reflections whose parameters differ from those of adjacent facies units. Seismic facies analysis describes and interprets seismic reflection parameters, such as abundance, configuration, continuity, amplitude, interval velocity and frequency, within the stratigraphic framework of a depositional sequence.
Direct seismic facies interpretation is to point out the geological causes responsible for the seismic signature of a seismic facies unit (lithology, fluid content, porosity, relative age, overpressure etc.). Indirect interpretation is to find answers with regard to the depositional environment and processes, such as sediment transport direction, transgression, regression, subsidence, uplift and erosion.
Identification of the seismic reflector geometries is the first step towards facies classification and provides information about depositional processes. It is perhaps the most useful for calibration with lithofacies interpreted from well logs, cores, and cuttings
Common examples of seismic reflections; from: Seismic Facies Classification – SEG Wiki.
Seismic facies interpretation of a prograding shelf in the Santos Basin of Brazil; source SEG Wiki.
Strongly related to seismic facies mapping is (seismic) sequence stratigraphy. Seismic sequence stratigraphers deliver a chronostratigraphic framework of genetically related stratigraphic surfaces, as wells as high quality regional- to prospect scale seismic facies and environment of deposition (EOD) assessment of the sediment bodies, bounded by these surfaces. The ultimate deliverability is the identification of prospective areas and in prospect evaluation, which is in support of prospect generation and well location selection to the Client’s team. OPS OES Thailand will provide industry experts. More details are given under Sequence Stratigraphy.
All terms in the diagram refer to relative sealevel and sedimentation rates/accommodation space. (source: SEPM Stratigraphy web).
Seismic Pore Pressure Prediction
Since seismic velocities correlate with effective pressure in the formation, sufficiently precise estimates of velocity obtained from seismic observations can be used to determine pore pressure across reservoirs, fields, and even at the basin scale, in particular after calibration with pore pressure analysis from well data. In the absence of dense well control, interval velocities derived from stacking velocities are used to estimate pore pressure. These interval velocities are compared with a general trend of velocities in the region. The general principal is explained in the below diagram.
Seismic pore pressure prediction volumes can be used to help with well engineering decisions such as trajectory and completion planning. Pre-drill knowledge of areas of potentially anomalous over- or under-pressure can help to mitigate drilling hazards. OPS OES Thailand geophysicists are all very familiar with the technique, and will, during the well planning process, advice the drilling and reservoir engineering departments.
Seismic pore pressure principle. Source: Petrowiki
AVO analysis is a seismic attribute, a technique that geophysicists can execute on seismic data to determine a rock's fluid content, porosity, density or seismic velocity, shear wave information, fluid indicators (hydrocarbon indications). AVO can also be known as amplitude versus angle (AVA), but AVO is the more commonly used term because the offset is what a geophysicist can vary in order to change the angle of incidence. (See diagram)
AVA/AVO behavior in pre-stack seismic gathers is a classic direct hydrocarbon indicator (DHI); variations in amplitude of a seismic reflection with angle of incidence or source-receiver distance (offset) are caused by fluid and/or lithology changes in the reservoir (see example below).
(source: SEG Wiki).
Fault Seal Analysis
In areas with an active tectonic past, the analysis of the sealing properties of faults is an important aspect both for the understanding of HC migration pathways from the source rock into the trap and for the evaluation of the internal trap structure and integrity. The understanding of the sealing capacity of faults, or cross-fault juxtaposition of strata, will mitigate the exploration risk. The fault seal analysis is usually a combination of seismic and well data analysis. The industry is utilizing several software platforms to help both geologists and geophysicists.
Parameters Juxtaposition and SGR extracted from the geomodel along the fault plane (From: Klarner S. et al.: Fault Seal Analysis from seismic and well data. Conference Paper EAGE Far East Hydrocarbons. Volume: Yuzhno Sakhalinsk)