Agreement DE-FC26-02NT15342, Seismic Evaluation of ...

the pressure trends were analyzed, as were rock and fluid properties for brine and
hydrocarbon saturated cases. Finally the Ursa field depositional setting was compared to
the Brushy Canyon depositional setting to establish the commonalities between the two
deepwater settings. These observations and reservoir properties are essential in
developing geologically realistic models that will be discussed in the next chapter for
forward modeling.
Forward modeling
Forward modeling was performed on six geologic models built to represent three
different reservoir intervals. The first reservoir interval is the Lower yellow reservoir
interval, which will have two geologic models; an amalgamated sheet sand and a basin
floor channel system. The second reservoir to be modeled is the Above Magenta
reservoir, which will be modeled with an amalgamated and non-amalgamated channel
complex. Finally, the Upper reservoir interval will be modeled with proximal and distal
levee overbank facies. These reservoir intervals were chosen because they can be
associated with high amplitude events on seismic data and each one represents one of the
three common reservoir facies mentioned above (sheet sand, channel fill and
levee/overbank). Each of these six geologic models were saturated with brine, oil, 10%
and 50% gas respectively. The models were calibrated with the Ursa reservoir fluid and
rock properties. Forward modeling was performed using a simple convolution between
the impedance volume and extracted wavelet was preformed on each model. Also,
Amplitude Versus Offset (AVO) modeling using the Zoeppritz equation was preformed
on one pseudo well for each model. A total of 24 separate 2D synthetic models and 24
AVO models were made, one for each reservoir facies and fluid case. For each reservoir
interval that was modeled the two possible seismic facies interpretations were compared
to determine if the lithology and fluids could be differentiated. This chapter will describe
the method of developing the reservoir models and rock and fluid properties from the
reservoir interval. This chapter will also give a general summary of the method used to
develop each of the models and precedes the three chapters that will discuss each
reservoir and its models and results in detail.
Example of forward modeling method
To generate the reservoir model, first the geologic model was created with an
image file. This image was created as an 8-bit grayscale bitmap image which with a total
of 256 possible colors associated with it where each color is represented by a number,
black being 0 and white 255. The image is then scaled to the size of the reservoir by
converting pixels to meters. This calibrated image is then output as a digital file using the
United States National Institute of Health (NIH) imagej freeware
(http://rsb.info.nih.gov/ij/). Each number/pixel represents a location in the reservoir
model. Not every pixel was used to model the reservoir. Instead the model was sampled
at 50 meter lateral spacing to match the Ursa seismic trace spacing. A matlab code
(Appendix D) was written to sort the imajej output text file into columns that would
eventually represent pseudo well logs spaced 50 m apart. Once these new 50-meter
spaced bitmap pixel numbers were sorted, they were placed into an excel spreadsheet and
converted to proper Vp, Vs, and density values associated with the reservoir interval
being modeled. For all models, the horizontal scale was developed using the depositional
history of the reservoir interval. The vertical scale was developed using the Ursa number
Agreement DE-FC26-02NT15342, Seismic Evaluation of Hydrocarbon Saturation 83