The Petroleum System Concept

Introduction The petroleum system is a unifying concept that encompasses all of the disparate elements and processes of petroleum geology. Practical application of petroleum systems can be used in exploration, resource evaluation, and research. This chapter discusses its application to petroleum exploration.

What is a petroleum system?A petroleum system encompasses a pod of active source rock and all genetically related oil and gas accumulations. It includes all the geologic elements and processes that are essential if an oil and gas accumulation is to exist.Petroleum describes a compound that includes high concentrations of any of the following substances:

o Thermal and biological hydrocarbon gas found in conventional reservoirs as well as in gas hydrates, tight reservoirs, fractured shale, and coal

o Condensateso Crude oilso Natural bitumen in reservoirs, generally in siliciclastic and carbonate

rocks

System describes the interdependent elements and processes that form the functional unit that creates hydrocarbon accumulations.

Elements and processesThe essential elements of a petroleum system include the following:

o Source rocko Reservoir rocko Seal rocko Overburden rock

Petroleum systems have two processes:o Trap formationo Generation–migration–accumulation of hydrocarbons

These essential elements and processes must be correctly placed in time and space so that organic matter included in a source rock can be converted into a petroleum accumulation. A petroleum system exists wherever all these essential elements and processes are known to occur or are thought to have a reasonable chance or probability to occur.

Petroleum system investigationA petroleum system investigation identifies names, determines the level of certainty, and maps the geographic, stratigraphic, and temporal extent of a petroleum system. The investigation includes certain components:

o Petroleum–petroleum geochemical correlationo Petroleum–source rock geochemical correlationo Burial history charto Petroleum system mapo Petroleum system cross sectiono Events charto Table of hydrocarbon accumulationso Determination of generation–accumulation efficiency

Identifying a Petroleum System

Introduction

Before a petroleum system can be investigated, it must be identified as being present.

Petroleum system identification

To identify a petroleum system, the explorationist must find some petroleum. Any quantity of petroleum, no matter how small, is proof of a petroleum system. An oil or gas seep, a show of oil or gas in a well, or an oil or gas accumulation demonstrates the presence of a petroleum system.

Procedure: Identifying a petroleum system

The table below outlines the steps required to identify a petroleum system.

Step Task

1 Find some indication of the presence of petroleum.2 Determine the size of the petroleum system by the following series of

steps:a Group genetically related occurrences of petroleum by using

geochemical characteristics and stratigraphic occurrences.

b Identify the source using petroleum–source rock correlations.

c Locate the general area of the pod of active source rock responsible for the genetically related petroleum occurrences.

d Make a table of accumulations to determine the amount of hydrocarbons in the petroleum system and which reservoir rock contains the most petroleum.

3 Name the petroleum system.

Naming a Petroleum System

Introduction

A unique designation or name is important to identify a person, place, item, or idea. As geologists, we name rock units, fossils, uplifts, and basins. The name for a specific petroleum system separates it from other petroleum systems and other geologic names.

Parts of a petroleum system name

The name of a petroleum system contains several parts that name the hydrocarbon fluid system:

1. The source rock in the pod of active source rock2. The name of the reservoir rock that contains the largest volume of in-

place petroleum3. The symbol expressing the level of certainty

Basin and Petroleum System Modelling

Basin and petroleum system modelling (BPSM) reconstructs the deposition of source, reservoir, seal and overburden rocks and the processes of trap formation and hydrocarbon generation, migration and accumulation from past (left) to present (right).

Fig: Simulating geologic, thermal and fluid-flow processes in sedimentary basins over time

Comprehensive modelling software incorporates data to simulate the interrelated effects of deposition and erosion of sediments and organic matter, compaction, pressure, heat flow, petroleum generation and multiphase fluid flow.

In essence, basin and petroleum system modelling (BPSM) tracks the evolution of a basin through time as it fills with fluids and sediments that may eventually generate or contain hydrocarbons.

In concept, BPSM is analogous to a reservoir simulation, but with important differences. Reservoir simulators model fluid flow during petroleum drainage to predict production and provide information for its optimization. On the other

hand, BPSM simulates the hydrocarbon-generation process to calculate the charge, or the volume of hydrocarbons available for entrapment, as well as the fluid flow, to predict the volumes and locations of accumulations and their properties.

Basin and petroleum system modelling brings together several dynamic processes, including sediment deposition, faulting, burial, kerogen maturation kinetics and multiphase fluid flow.

Basin and petroleum system modelling consists of two main stages: model building and forward modelling. Model building involves constructing a structural model and identifying the chronology of deposition and physical properties of each layer. Forward modelling performs calculations on the model to simulate sediment burial, pressure and temperature changes, kerogen maturation and hydrocarbon expulsion, migration and accumulation. Calibration compares model results with independent measurements to allow refinement of the model.

Fig: The multiple and interrelated steps of BPSM

Logging

Wireline logging can be used in a number of ways by a number of people to provide solutions to questions they have about a particular well. Some of the ways different people in an office will use these logs are:

Geophysics look to logs for:–Where are my tops (as predicted?)–Does seismic interpretation agree with log data?–How is my synthetic doing with this new information?

Geologists look to logs for:–Where are my tops?–Do I have any reservoir?–Is there any Hydrocarbon in the well?–What type of Hydrocarbon(s) is there?–How good is my reservoir?–What kind of reserves do I have?–How does this tie in to my offsets?

Drilling Engineers are looking for:–What is my hole volume (cement)?–What is my dog leg severity?–Where can I get a good packer seat for testing?–Where can I set up my whip-stock?

Production Engineers are looking for:–Where should complete this well?–What will be my expected production rates?–Will I have to deal with water?–How should I complete this well?–Do I need to stimulate this well?–How should I stimulate it?

Bentiu 1 (1228-1232 mKB) Test#4 (1228-1232 mKB)Swab 44 bbl oil with 90% water cutNet pay 3.1m 30% Sw 38%

Test#3 (1393.5-1400 mKB)Swab 91.4 bbl of oil with 50% water cutAPI 37.1°Net pay7 m 25% Sw 22%

Bentiu 2 (1393.5-1403 mKB)

WELL BY WELL REVIEW

No. of wells in the field Categorise all wells in the following manner

If a well is producing at less than 40 BOPD without nay above reason, should be closed.

Q< 100 BOPD, work-over can be recommended. For Q<100BOPD wells and EFFECTIVE IDLE wells need a clear plan.

Generally going for effective idle well to work over instead of producing wells. They are economically favourable.

INFORMATION GATHERING:

A.GEOLOGICAL DATA 1.Location map of well in problemZones (like aredebia, bentie, abugabra)

WELL

PRODUCING NON-PRODUCING

Q > 100 BOPDGOOD WELL

Q < 100 BOPDSICK WELL

IDLE WELL DEPLETEDEnergy is down having no more potential to

produce, high WC/GOR

NON-EFFECTIVE WELL

Well having mechanical problem like

Need fishing Casing parted etc.

EFFECTIVE

1. High water cut2. High GOR3. Sand producing

Note:-

If up-dip well is not producing then down-dip well may not produce If down-dip well is producing then up-dip well will definitely produce. For two nearby wells on same elevation, if one is not producing then

there may be fault in between them.

2. LOG DATA ( Wire-line and Mud (master) log)

Petro physical surveysTop-bottomPorositySaturationPermeabilityGross pay thicknessNTG ratioNet pay thickness

If a new zone in being looked for opening then details of petro-physical data will be required. Sometime it is better to consult petro-physicist (experienced person).

Location of OWC, ODT, WUT etc.From resistivity curve (LLS, LLD, MSFL)

3. Cross section (X- section) / well correlation To show sand continuity Important for IORTo check performance from nearby wells

B. Reservoir data from R.E

1. Well testing report Well testing reports are available only for exploratory and/or appraisal well. Generally tests are not allowed to conduct for development wells as not economically favourable. But sometime PLT (Production Logging Tool) is conducted for development well if oil has been found in exploratory well and another well is not producing.

2. Well historyWell history is very necessary to understand the particular well. It includes work-over jobs (WOJs), completion date, recompletion date, any pump installation etc. After every work-over a final well report is prepared.

BOTTOM’s UP STRATEGY FOR COMMINGLE COMPLETION

Completion

When a well is drilled through pay zone then it is cased, cemented and perforated to connect the reservoir to surface facility through well bore. It facilitates a pathway through damaged (invaded) zone. To extract the reserve from reservoir it is recommended to complete the well very carefully to prevent early water production.

Commingle Completion

When more than one pay zone are completed together called commingle completion. Completion is made for the following reason-

Depletion strategy To maximise oil recovery To recover investment Cost reduction For multilayer reservoir

Along with advantages it have some disadvantages Cross flow Very difficult to understand each reservoir individually (difficult to

collect PVT sample). In case of sand/ water production it is very difficult to control.

BOTTOM-UP STRATEGY

BOTTOM-UP strategy is to complete the well from down to upward direction. This strategy is to maximise recovery with minimum investment. “If structurally lower (down dip) well is producing then there is much more chances that structurally up (up dip) well will produce. But if structurally up (up dip) well is producing oil then structurally lower (down dip) well may/may not produce oil.” So while development it is advisable to exploit from bottom to up direction i.e bottom first and/or along with upper zones.

Say a reservoir is having three zones. If zone 1 is produced from top then while abandon it will be squeezed and will need to be drilled again to open the zones beneath the 1st zone. In simple note this strategy is to-

Save time of completion Minimises investment required for completing a multilayer. Helps in recovering maximum reserves. Sometime lower zone producing high water cut can be closed just placing

a DBP (Drillable Bridge Plug) or RBP (Retrievable Bridge Plug)

Case study: