GEOLOGICAL SERVICES

Well Correlation and Mapping

Both in the exploration – and field development phase, well correlation is a principal instrument for the geologist to understand the stratigraphy in a basin, or through a field. Well correlation also takes place at reservoir bed scale (layering of the reservoir).

Our geologists are experienced in the correlation application in Petrel, OpenWorks, or another industry software platform. An example of a well log correlation is given in the diagram below.

Source: researchgate.net

In areas where reservoirs are relatively small, such as in the Gulf of Thailand gas fields, detailed well correlation is extremely important to understand if, and which reservoirs in adjacent wells, are correlatable, in view of completion and development. OPS OES’ geologists have a wealth of experience in well correlations, from many parts of the world.

Equally important is the use of seismic data in well correlation, given proper well-seismic ties. The geologists we provide have also a wide experience in using biostratigraphy as a correlation and mapping tool.

Our geologists are very experienced mappers, using several industry- standard software mapping platforms.

Well Log Interpretation

Well log interpretation is an essential method for the geoscientist to acquire more knowledge about the subsurface formations and conditions by using physical properties of rocks. It is essential to detect hydrocarbon-bearing zones, calculate the hydrocarbon volumes, and many other parameters.

Although the quantitative petrophysical log interpretation and calculations are usually handled by the petrophysicist, the geologist must have an extended basic knowledge of well log interpretation. The geologists provided by OPS OES all have a solid basic understanding and wide experience in qualitative interpretation of electric logs in wells.

Basic Well Log Interpretation (Source: slb.com).

All our geologists are able to quickly identify reservoir sections (sand or porous limestone) and whether these are hydrocarbon bearing or not, as well as the average porosity and saturation of these zones. They will also quickly identify oil pay versus gas pay and be able to identify fluid contacts. As such, the project geologist is perfectly capable of having meaningful petrophysical reviews with the petrophysicist.

Sedimentological Core Description and Analysis

Sedimentology is the study of modern sediments such as sand, silt, and clay, and the processes that result in their formation (erosion and weathering), transport, and deposition. It also deals with the physical and chemical changes that occur when sediment is converted into sedimentary rock (diagenesis). Sedimentologists apply their understanding of modern processes to interpret geologic history through observations of sedimentary rocks and sedimentary structures. Sedimentary rocks can be classified as: clastics, carbonates, evaporites, and chemical.

OPS OES provides sedimentologists who have wide-ranging expertise in close examination of rock in core- sidewall core- and drill cuttings material and integration with other geosciences. This integration has tremendous applications in high-resolution reservoir characterization studies and predictive modeling in field development and potential productivity. The application in exploration lies mainly in the identification of new opportunities by understanding the spatial sedimentological relationships in an area or basin.

Continuous cores from wells provide very good data for stratigraphic correlation, interpretation of depositional environment; reservoir geometry, reservoir quality (porosity, permeability) and wireline log calibration. When examining core material, our sedimentologists prepare a graphic log, which visually expresses a stratigraphic succession and usually reflects the following (see diagram):

Thickness
Grain size
Sedimentary structures
Accessories, such as fossils and diagenetic features (clays)
Lithologies and nature of contacts between different lithologies
Textural maturity
Oil staining
Fracturing
Porosity

Example of a sedimentological graphic log

Core plugs provide samples for further analysis of reservoir characteristics, such as porosity, relative permeability, fluid saturation, capillary pressure, wettability, as well as electrical characteristics to understand resistivity, formation factor and cation-exchange capacity, and petrographic studies. This special core analysis goes beyond the qualitative core description and is executed in the lab.

EOD; Facies Mapping; Paleogeography

Geologists always are curious in what geological environment the sediments encountered in the well, were deposited. Cuttings-, core data and well logs give a good picture of the lithofacies and help the geologist in the interpretation of the environment of deposition (EOD). Equally helpful is the utilization of paleontological data, which not only gives insight in the age of the horizons, but also on the water depth and the EOD. With the aid of sufficient wells with paleontological data, our geologists are very well capable of mapping the paleogeography of the strata through time.

Another great tool in constructing palaeogeographical is (seismic) sequence stratigraphy, which enables the geophysicist and the geologist to map the EOD of an area or basin where no or very limited well data is available. Seismic sequence stratigraphy models rock bodies that are interrelated through space and time (e.g. delta-turbidite sequences). Favorable sites for deposition of reservoir, seal, and source rocks can be ascertained from palaeogeographical maps. For instance, it will enable the geoscientists to map sand-rich turbidites on deep-water basin slopes, and as such, it is a good way to mitigate exploration risk.

Example of a seismic facies map and corresponding palaeogeographical map of the Middle Miocene Taranaki Basin, offshore western New Zealand (Bally, A. W., ed., 1987, Atlas of Seismic Stratigraphy: AAPG Studies in Geology 27, vol. 1, 124 p.).

Regionally, many examples of palaeogeographical maps are well-known from the Indonesian waters of the Makassar Straight, offshore Thailand in the southern Andaman Sea and in the Moattama Basin, offshore Myanmar.

Hydrocarbon-In-Place

Our geologists are all highly experience in the mapping and estimate of hydrocarbon-in-place. As they have worked in places with large continuous reservoirs (such as in the North Sea), and with small scale, discontinuous reservoirs (as in the Gulf of Thailand), they can execute both deterministic HCIIP mapping, as well as stochastic resource estimation. In areas with sufficient well control over a field or reservoir, the petrophysical evaluation in the wells, will provide the parameters to construct hydrocarbon pore feet maps. These can be constructed by hand, where the geologist can put in his “geological imagination”, or in one of the industry standard software platforms (e.g. Petrel). This will result in the STBO or SCFG at surface conditions. This widely accepted methodology couldn’t be used in areas with small, discontinuous reservoirs. However, thanks to the (usual) huge amount of wells (often 16 or more per platform) and multiple platforms, it is possible to make an estimate of the resources. Normally, the estimation per platform is far more accurate than the estimation per well. For the stochastic methodology, many Clients have their in-house software.

Technical Studies

There is a wide range of technical studies whereto OPS OES can provide very capable technical professionals. These studies vary from basin-wide petroleum potential evaluations, detailed reservoir studies such as origin and distribution of illite, by-passed hydrocarbons, gas composition studies (thermogenic versus biogenic), structural-stratigraphic framework reviews in new basins. For highly specialized studies, such as Amplified Geochemical Imaging, we will provide experts from the global market.

Well Planning

The objective of well planning is in essence three-fold: it must be safe, drilled at minimum cost and it must be usable. Planning a well is an iterative process between geoscience, reservoir- and drilling engineering staffs and involves frequent accessing of several databases and clear communication. Three basic areas need to be examined in planning a well:

What target(s) will be evaluated?
How will the well(s) be drilled to reach those target(s)?
How will the target zone(s) be evaluated?

The development geologist plays a major role in well planning and often acts as the liaison between the various departments, both during the pre-drill-, the drilling-, and the post mortem phase. Among many others, major responsibilities for the geologist are:

Prediction of the geological column to be drilled
Pore pressure prediction in relation to hydrostatic pressure
Casing depth selection
Planning a vertical (most exploration), deviated or horizontal well as optimized well trajectory
Data Collection (Logs, cores etc.)

OPS OES geoscientists are experienced well planners both in exploration and field development, including multiple horizontal wells.

Integrated Reviews and Studies (FDP)

OPS OES will provide a multidisciplinary team of at least one geophysicist, one geologist, one reservoir engineer and a drilling engineer. This team can be extended with other experts when required. In case of a single field review or a full-scale Field Development Plan (FDP), usually a static reservoir modeler (geologist) and a dynamic modeler (reservoir simulator) will be part of the team.

The “ideal” Field Development Plan Workflow is summarized as follows:

Development and Depletion Strategies
Environmental Considerations
Data Acquisition and Analyses
Geological and Numerical Model Studies
Reserves and Production Forecast
Facilities Requirements
Economic Optimization
Management Approval

In such a multidisciplinary team, the development geologist will be driving geological interpretations on various hydrocarbon accumulations with different structural-stratigraphic frameworks. The development geologist is a key team member, who is also co-responsible for resource estimation and geological risking, and will be assisting in development well planning, optimization and execution of the development program.

Prospecting

Prospect generation and evaluation is an area of exploration in which hydrocarbons are predicted to exist in economic quantity. A prospect is commonly an anomaly, such as a geologic structure or a seismic amplitude anomaly that has been mapped, but not drilled yet. A group of prospects of a similar nature constitutes a play. A structural lead becomes a prospect once we determine that the five geological elements of the petroleum system have sufficient probabilities of success in contributing to an economic accumulation of hydrocarbons in the structure. Prospects could either consist of structural traps or stratigraphic traps.

The process of prospecting usually starts at basin scale and subsequently zooms in to the individual prospect level, as illustrated above.

Teams provided by OPS OES have the ability to thoroughly assess prospects and leads and convert leads into economically drillable prospects. They also are very experienced in estimating the amount of (recoverable) hydrocarbons.

Geological Risking (POS)

Both exploration- and development geologists are heavily involved in pre-drill assessing the chance (probability) of geological success (POS).

It is common knowledge in the industry that 5 geological factors determine if a well (exploration, appraisal, step-out) is a geological success or a dry hole:

Source Rock
Reservoir Rock
Seal or Cap Rock
Trap
Migration (Hydrocarbon Charge)

All five geological elements need to work; if one fails, the well will be a dry hole. The overall POS is achieved by multiplying the probability of the presence and effectiveness of these five elements:

POS = P Source * P Migration * P Reservoir * P Trap * P Seal.

The five elements in geological risking. (Alan Foum, Consulting Geophysicist, published on LinkedIn on 5 July 2018).

The probability number will range from 1.0 (certain) to 0 (not possible). Each Operator uses its own very descriptive matrix as a standard in geological risking, but the differences are usually not large. The risking exercise is a multidisciplinary team effort, and will be usually peer reviewed before being finalized. Generally the geologists risk the probability of finding the estimated volumetric range (normally given as a probabilistic estimate P90 to P10; commonly companies risk to find the P50 reserves), this is the geological chance of success. All OPS OES geoscientists have a sound understanding of and wide experience in applying the methodology from different Company matrixes.

By-passed Oil (PLT, RST review)

The RST Reservoir Saturation Tool combines the logging capabilities of traditional methods for evaluating saturation in a tool slim enough to pass through tubing. Determining hydrocarbon and water saturations behind casing plays a major role in reservoir management. Saturation measurements over time are useful for tracking reservoir depletion, planning workover and enhanced recovery strategies and diagnosing production problems such as water influx and injection water breakthrough. Moreover, it provides a log of the borehole oil fraction, or oil holdup, even in horizontal wells. Our geologists have routinely utilized the RST to detect by-passed oil in “depleted” reservoirs or in partially depleted fields.

Studying and characterizing the reservoir during production can play a vital role in improving reservoir performance and production optimization. Production logging tools (PLT) are developed to assist with allocation of production to different zones as well as diagnosing the production problems such as leaks or cross flow. Our geologists and reservoir engineers are very familiar with the PLT interpretation.

Fault (plane) Mapping

A fault plane is the plane that represents the fracture surface of a fault. Usually, faults are well defined on seismic data. However, under certain conditions, faults cannot be recognized and interpreted from seismic data, e.g. due to a gas plume throughout the section obliterating the seismic signature, such as in the Attaka Oil Field in East Kalimantan. In this case, the geologist needs to contour the fault plane from wells intersecting the fault at different depths (illustrated in the below diagram). Fault plane mapping is a fundamental tool for prospect evaluation, in particular in areas with no seismic control.

Source: Sciencedirect.com

A major application is the assessment of the fault seal risk in prospects, by constructing cross-fault juxtaposition cross sections. Using depth structure maps, the top of each mapped stratigraphic unit is projected at its intersection with the fault plane to its correct depth on the fault plane profile. Fault plane profiles are important, as they show what is being juxtaposed across the fault and thus help defining leak points.

Example of a fault plane profile, showing juxtaposed lithology leak points.

OPS OES geologists have a solid understanding of the concept of fault plane mapping due to their experience in working in different areas around the globe.

Production Applications

Before bringing an oil- or gas field on-line, the Operator must submit a Production Application to the authorities and copy all stakeholders in a license block. This is a comprehensive report, which contains the drilling history of the field, the reserves estimation, the development planning and the recovery. It should also include an Environmental Impact Assessment (EIA) and an economic section. Basically, it is a complete collection of all available data and must contain all wells, pay counts, reserves, and future development. All geologists we provide are very capable of writing excellent PA’s and presenting these to the Government.

RichardJ@opsoes.com