David A. Ford

Deep Wells in Deep Water: Gulf of Mexico Rock Property Observations / AVO Implications

Rock Property trends observed in deep wells in deep water Gulf of Mexico define AVO responses, which deviate from conventional (shallow or mature) Gulf of Mexico responses. While the frequency of atypical AVO responses for pay sands is only 16%, the frequency of unexpected AVO responses for brine sands is 62%. Thus, the use of conventional techniques to differentiate hydrocarbon from wet sand reservoirs is compromised in deep water.

Method for generated log parameter volumes from seismic

Wire line logs have been used by geologists, geophysicists and reservoir engineers to identify parameters such as lithology type, fluid type, degree of invasion, degree of hydrocarbon saturation, and porosity of subsurface formations. The reliability and accuracy of well logs are such that they are considered ground truth. The limitation of logs as a tool for understanding the subsurface is, of course, that a hole must have been drilled in which the logging tool can be run, limiting its use to a post drilling evaluation tool rather than allowing it to be used as a predrill exploration tool such as seismic. As an evaluation tool, the sparse surface sampling resulting from the measurements restricted to a well bore, although offering excellent vertical sampling, is limited compared with seismic which offers dense surface sampling but has, compared with logs, limited vertical sampling. The disadvantages of seismic are that amplitudes and attributes are generally not considered as directly correlated with the important geologic and geophysical parameters routinely derived from logs and they are not as easily and accurately interpreted.

Quality controlling AVO inversion products: Examples of amplitude decomposition into rock property contrasts

Rock property contrast cubes derived from AVO inversion have brought the industry closer to extracting the physical properties of the subsurface from seismic data. VanKoughnet et al. (2002) showed the value of this type of application to complex reservoir problems. This study takes on the challenge of demonstrating the potential accuracy and existing pitfalls of this technique through a process termed amplitude decomposition. Examples derived from both wireline and real seismic data will be used to demonstrate the technique and support the conclusions.

P-S AVO Attributes And Cross-Plotting

Summary P-P AVO has proven to be one of the most effective geophysical risk reduction methods available. With the widespread availability of multi-component data and improved multi-data type true amp processing methods it becomes possible to accurately calculate PS AVO attributes. These attributes, used alone or jointly with P-P attributes, provide a simpler, more direct connection to the underlying rock property contrasts resulting in quantities which are easier to interpret and more sensitive to fluid type, saturation and reservoir quality variations. Cross-plotting multiple attributes provides even more discrimination leverage and sensitivity than traditional plotting methods. It will first be shown that Equation 1 provides a linearized form of the exact equation. This form is accurate for most exploration cases. The D0 vs. D1 cross-plot displacement signatures associated with each rock property contrast will be shown in Figure 1. In addition, it will be shown that the crossplot background trend (BT), associated with reflections from layers following local average rock property trends, can be calculated from simple relationships. The cross-plot displacement from the BT, associated with hydrocarbon substitution, can then be easily predicted. It will also be shown that P-S attribute cross-plots can provide information about the variation of rock properties associated with two spatial locations or for a single location but at two times (T1 and T2) if time lapse data is used.

The Interpretation of P-P AVO Cross-Plots

AVO attributes, when examined using traditional color plots, have proven to be one of the most effective exploration risk reduction tools available. Cross-plotting multiple attributes provides even more discrimination leverage and sensitivity, yet its use and appreciation has been limited because of the widespread lack of understanding about how to interpret cross-plots and take full advantage of the large amount of information contained within them. It will be shown that each rock property contrast has a characteristic cross-plot signature (as shown in Figures 1, 2 and 3), and that the cross-plot background trend (BT), associated with reflections from layers which follow local average rock property trends, can be calculated from simple relationships. The cross-plot displacement from the BT associated with hydrocarbon substitution can then be easily predicted. With reasonable assumptions, the cross-plot displacements associated with two spatial locations can be related to the contrast in the reservoir’s rock properties between the two locations. This allows a consistency check against the scenario of a portion of a reservoir filled with brine while the rest is filled with hydrocarbons of various types and saturations. Finally, the cross-plot displacements associated with the attribute values measured at times T1 and T2, but at a single location, can be used to indicate reservoir sweep efficiency and the location of bypass zones. The common BT for the two can be used to calibrate the time lapse AVO attributes.

A public‐domain program for viewing 3‐D data

The enhancements to the functionality of an existing program developed at the Colorado School of Mines Center for Wave Phenomena that is now used to view movies of seismic data will allow for a more flexible way to view data of any type. These enhancements turn the Seismic Unix processing system program SUXMOVIE into a more modern interactive 2-D and 3-D viewing tool that can be added to in the future by the large group of users that already support SU.