Environmental and Exploration Geophysics II tom.h.wilson Department of Geology and Geography West Virginia University Morgantown, WV Introduction to the Granny Creek Seismic Exploration Project

Exam will cover elementary reflection and refraction seismology including 1) the 2-layer reflection and refraction analysis, 2) 3 layer refraction analysis 3) forward and reverse refraction profiling (single layer) 4) energy loss mechanisms and energy partitioning

Normal Incidence Seismic Profile Shot point locations TwowaytraveltimeTwowaytraveltime

The exploration project area is located in the foreland of the Appalachian Mountains

Faults in the basement play an important role in controlling the location of hydrocarbon reservoirs.

This seismic profile crosses the Rome trough and reveals several basement faults

Distribution of sediments in the basement cover with interval velocities and traveltimes.

Interpretation of major reflection events observed in the foregoing seismic section

Note the subtle thickening and thinning of the section across the line.

Note that layer 1 thickens over the trough indicating that the trough was subsiding during the deposition of those sediments. Layer 4 however, thins out across the trough indicating that the floor of the trough must have been uplifted during the deposition of that layer of sediment

In this seismic section you can see that to the left in the deeper part of the section reflectors drop down to the left, but rise to the left in the shallower part of the section. Downward movement along the fault evident in this section reversed later in time. Later in the history of the area basement block on the left moved upwards.

When we examine various regions within this area we see that that faulting of the basement surface was not confined to a single episode of deformation but continued through time.

We can plot the reflection events in a relative rather than absolute depth scale to enhance some of these subtle structures.

Reversal of fault motion can be observed throughout the regions. The example below comes from one of the lines to the north.

The correlation of reflections to subsurface stratigraphic intervals is made using synthetic seismograms compiled from well data obtained near a seismic profile. We’ll talk more about synthetics in the weeks to come.

You’ll be looking at seismic data from the Granny Creek oil field in Clay and Roane Counties.

Shumaker, 1995

Production is from the “Big Injun” sands. These sands are Mississippian in age (~ 335 million years old). Zou, 1993

Several seismic lines are available across the Granny Creek field. You’ll be interpreting 5 of these lines as part of this exploration project.

Several wells have been drilled in the field and this map shows the variations in cumulative oil production throughout the field.

This line courses through the field from its southeastern to northeastern extents. Line 6 from your handout

Prominent reflection events have been identified in this slice of seismic traces.

In this display, line 6 has been reversed, so that the northwest end is off to the right. Note the general structural style and the reversal of movement that occurred later in the history of the area elevating the shallower strata to the northwest.

This is a contour map of sub-sea depth to the top of the Big Injun sands. Note that the field coincides with a structural low that trends northeast-southwest.

This map of the basement faults beneath the area was compiled from seismic data. The fault to the east or left in this map marks the east margin of the Rome trough. The fault to the west drops down into the trough interior.

Again note the east margin of the trough and the second step out.

The synthetic seismogram mentioned earlier can be used to provide a detailed view of individual reflection events and their relationship to the detailed stratigraphic features in the reservoir and surrounding intervals.

These synthetics indicate that reflections from the top and base of the reservoir interval are mixed in with reflections from surrounding intervals, but coincide with the negative cycle observed at about 0.31 seconds.

Different types of seismic sources were used to collect seismic data over the Granny Creek field. This seismic line was generated using a weight drop for the source.

Remember our discussions near the beginning of the semester. The greater the range of frequencies in the seismic pulse, the sharper the pulse becomes. The sharper pulse provides higher resolution of subsurface intervals

This map shows the variation of travel time from a shallower reflector to the Big Injun. Note that during that through the more productive northern part of the field, travel times are less, implying in this case that the interval has thinned.

In this map the subsidence trends at various times in the past are plotted relative to areas of high cumulative production.

In this area we see that a series of basement faults formed early in the history of the basin.

As time passed (Ordovician to Pennsylvanian) we see that minor movements along these deeper structures created subtle patterns of thickening and thinning in deposited strata.

We also infer the presence of late stage detachment resulting from compression forces associated with plate collision during the formation of the Appalachian Mountains. In addition, it appears that movement along the deeper basement faults may have continued after the episode of folding.

In the time remaining today read carefully through the exploration project instructions and begin interpreting Line 6. Next Tuesday and Thursday, I would like for you to work together as a group and decide how you will approach the analysis of this data and divide up the work on specific tasks you feel should be undertaken.

The two problems presented in class and problems 3-2, 3-3 and 3-4 from Burger are due next Tuesday.