Tom Wilson, Department of Geology and Geography Environmental and Exploration Geophysics II tom.h.wilson Department of Geology and Geography West Virginia University Morgantown, WV Traps and Prospects / Conversion to Depth / Complete the 3D Interpretation Workshop

Tom Wilson, Department of Geology and Geography To begin with, please copy the folder Golden-3 from the class common drive to your G:\drive. We’ll all be doing the same exercise today.

Tom Wilson, Department of Geology and Geography Reflection seismology unveils the subsurface for our inspection and interpretation

Tom Wilson, Department of Geology and Geography Essential ingredients needed to form hydrocarbon rich zones - source, reservoir, trap and seal

Tom Wilson, Department of Geology and Geography The explorationist at work

Tom Wilson, Department of Geology and Geography Sediments shed from the uplifted Sierra Madre Mountains pile up in coastal areas of the Rio Grande Embayment. The pull of gravity on this large mass of sediments caused faults to develop that accommodated gradual sliding or creep of large sediment laden blocks out into the Gulf of Mexico. Gulf Coast (Golden and BEG) Play

Tom Wilson, Department of Geology and Geography Deltas load the shelf with sediments and gravity takes over Sediments pile up in the embayment which slopes off into the Gulf of Mexico. Mass wasting of the shelf proceeded under the pull of gravity

Tom Wilson, Department of Geology and Geography Faults rise to the surface in the landward direction as the sediments take a sled ride into the Gulf. These faults accommodate extension at a slow (creeping) but steady pace. Time is always available in excess for the geologist.

Tom Wilson, Department of Geology and Geography As extension faults develop, strata collapse back into the fault plane and additional sediments fill the resulting void and additional faults dipping toward and away from the direction of movement – the synthetic and antithetic faults, respectively.

Tom Wilson, Department of Geology and Geography From Seismic to reservoir image

Tom Wilson, Department of Geology and Geography Seismic acquisition to subsurface imaging

Tom Wilson, Department of Geology and Geography Note the roll-over into the glide zone, synthetic and antithetic faults

Tom Wilson, Department of Geology and Geography

Complex traps and cap rock

Tom Wilson, Department of Geology and Geography Converting times to depth requires that you have velocity information. There are three different ways to come up with the velocities Depth = velocity * time In general you will have depths to formation tops derived from your log interpretations You will have travel time data from your seismic horizon interpretations & well surveys (checkshot and vertical seismic profile (VSP)). The checkshot and VSP data allow you to create a time-depth curve which can be used independently to convert any time to a depth or alternativel convert any depth to a time.

Tom Wilson, Department of Geology and Geography Conversion from time to depth Log picks TD Curves Horizon time picks

Tom Wilson, Department of Geology and Geography Three methods we’ll use: Apparent > 2*formation top depth/time from seismic horizon pick Time surface> 2* depth (from TD table)/time from seismic horizon pick (depth is determined from the TD chart for given horizon time). Formation top > 2*formation top depth/time from TD chart Average Velocity = (2 * Depth) / Two-way Time Average velocity approach

Tom Wilson, Department of Geology and Geography Apparent Velocity /Inverse Distance to Power From the compute average velocity map dialog help window. The depth in this approach is taken from the log picks In a 3D interpretation, you are likely to have horizon time picks and well formation top picks. This is just one approach

Tom Wilson, Department of Geology and Geography Apparent Velocity /Inverse Distance to Power The low in the southeast is anomalous. Bring up crossline 140 and have a look. The travel time to the interpreted C38 reflection is much higher than that to the well pick. The denominator is large and we have a small average velocity

Tom Wilson, Department of Geology and Geography

Time surface approach (with depth from TD curve) Well #13 is a deviated well. For this well, the total vertical depth (TVD) is erroneously high. The measured depth (MD) may have been used. Since velocity = depth/time, the resulting velocity is too high in this area.

Tom Wilson, Department of Geology and Geography Velocity map obtained without well #13

Tom Wilson, Department of Geology and Geography This depth converted map was constructed from the using the apparent velocity approach

Tom Wilson, Department of Geology and Geography Formation top approach (time from the TD curve)

Tom Wilson, Department of Geology and Geography Depth from Apparent velocity and Formation Top approaches

Tom Wilson, Department of Geology and Geography Depth conversion using time-surface approach

Tom Wilson, Department of Geology and Geography Depth Contour – two versions

Tom Wilson, Department of Geology and Geography Isochron Create time grid for each horizon & include your polygon set (i.e. GreenT or C38Time grids) Convert them to depth using your favorite velocity models Associate polygon sets with your grids Tools > Calculators > Math on two maps fine tune parameters and select one or the other polygon set We may not have time for this ….

Tom Wilson, Department of Geology and Geography In the end you have to ask yourself if the maps make reasonable geological sense and whether you can present a convincing argument in support of your interpretation.

Tom Wilson, Department of Geology and Geography Petroleum geology of the north sea: basic concepts and recent advances by Glennie (1998)

Tom Wilson, Department of Geology and Geography The seismic pick on the event interpreted as the Rodby is seconds. Times from seismic interpretations Note that the autopicking on the Rodby shown here was performed with little guidence just to help show where interesting faults and structures might be located and to help uncover predominant structural trends.

Tom Wilson, Department of Geology and Geography Depth pick on the Rodby is 2311

Tom Wilson, Department of Geology and Geography We can obtain two-way travel time to that depth using the TD function.

Tom Wilson, Department of Geology and Geography In the time-depth chart, there is a value for the time at a depth of feet of seconds. We interpolate to find the time corresponding to Rodby depth of 2311m From the TD function we estimate the time of for the Rodby

Tom Wilson, Department of Geology and Geography From the TD function we can also estimate a depth of from the horizon pick time of 2.056

Tom Wilson, Department of Geology and Geography Average Velocity = (2 * Depth) / Two-way Time Three methods: Apparent > 2*formation top depth (2* )/time from seismic horizon pick (2.056) = m/s Time surface> 2* depth (from TD table = m)/time from seismic horizon pick (2.056) = m/s Formation top > 2*formation top depth (2* )/time from TD chart (2.0698) = m/s

Tom Wilson, Department of Geology and Geography Average Velocity = (2 * Depth) / Two-way Time The three methods yield similar results in this case. Apparent > m/s Time surface> m/s Formation top > m/s

Tom Wilson, Department of Geology and Geography Another potential prospect

Tom Wilson, Department of Geology and Geography

Cutting loose the 3D Autopick

Tom Wilson, Department of Geology and Geography Autotracking fails at locations interrupted by local structure. These may be areas to explore further.

Tom Wilson, Department of Geology and Geography