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Reservoir Geology

Competency Statement:

Define a petroleum reservoir in terms of depostional environment, lithology, continuity and thickness of reservoir sands, and structural geometry to generate a geological reservoir model of the Gama Field. Estimate the original oil volume in place by using petrophysical parameters developed from well log analysis and the Stock Tank Original Oil In Place - STOIIP calculation.

Learning Objective:

  • Interpret well log, core, and pressure data for a petroleum reservoir model
  • Integrate depositional facies, their distribution, and the reservoir architecture into a geological model of the reservoir
  • Determine petrophysical properties of reservoir sands including Net to Gross ratio of net reservoir sands to total reservoir thickness, porosity, and oil or gas saturation
  • Estimate original hydrocarbons in place of the Gama Field

Assignment Instruction:

Beta Petroleum drilled the Gama 1 "wildcat" discovery well and three appraisal (delineation) wells (Gama 2, Gama 3 and Gama 4) on the Gama structure in the New Basin.

The discovery well found oil and gas in the Lower Graben Sand Formation but only defined the oil down to (ODT = lowest known hydrocarbons) depth and water up to (WUT = highest known water) depth. Gama 2, the 1st appraisal well, defined the oil/water contact (OWC). All of the wells have been logged and a whole core was taken of the reservoir in Gama 3. Gama 3 and 4 penetrate the same hydrocarbon bearing reservoir and display similar fluid contacts as Gama 1 and 2, but this needs to be verified using the available pressure information.

As the Exploration Geologist, develop a static reservoir model and determine the oil in place (Stock Tank Original Oil In Place - STOIIP) for the Lower Graben Sand Reservoir of the Gama Field:

1) Interpret flow units in and between each of the Gama wells to assess the reservoir properties across the Gama structure. The Lower Graben Sand can be sub-divided into 4 Flow Units each with different properties based on a type log from well F2-2A (Ref. 3448) located 8.6 km to the northwest. The top of the Lower Graben Sand is picked at the occurence of the first large sand beds below the marine shales of the Middle Graben Shale package.

2) Determine the local, or prospect depositional environment from well logs and with core data from Gama 3. Based on regional information, the Lower Graben Sand is interpreted as a fluvial-deltaic to a tidal delta depositional system, building out in a northern direction. The Lower Graben Sand is subdivided into four "Flow Units":

  • tidal distributary channel fills,
  • tidal channel bars,
  • channel margins and
  • lagoonal backwash systems.

3) Using the depositional environment, define the reservoir continuity of the individual permeable and non-permeable beds defining the “flow units” that control the direction of fluid flow through the reservoir into the producing wells. In the fluvio-deltaic/tidal distributary environment for this scenario, the width/thickness ratio of the sand bodies is 100 and the length/width ratio is 5.

4) To calculate the STOIIP, review the structure map of top of Lower Graben Sand in Gama Field (below and Ref. 3465) derived from well and seismic data. The structure is faulted, displays 4 way dip-closure, and appears to be filled to spill point. The crest of the structure is at 3025 m and is located northwest of the discovery well. The structure is broken up by two sets of normal faults, forming a NW-SE trending 1.5 km wide Triassic-age crestal graben. Younger north-south trending faults break up the eastern and northeastern flanks of the field. The throw of the faults varies from 10 - 85 m. The faults are partly sealing and may control some compartmentalization of the field. Variations in reservoir properties can be distributed across the structure to cover uncertainties in the subsurface model. However, in this module the average properties found in the wells will be used as input in the volumetric calculation of oil in place.

Specific information to complete the following assignments are listed in each Assigment Instruction and located in References and Field Data.

Structure Map of top of Lower Graben Sand in Gama Field