Seismic Data Processing and AVO Attributes Analysis

Marine Seismic Data

Total 6 marine seismic lines 92-6023, 83-50, 88-61-112, 88-61-217, 88-61-630, 92-6126 crosses the exploration well Gorae in block VI-1 of the Korea continental shelf and 2 land seismic lines from Kazakhstan were studied this year

Work processes done in this study for marine seismic data consist of seismic data processing for AVO, AVO analysis, AVO inversion and AVO modeling. The seismic line 92-6023 was used to describe detail processes.

In data processing, we didn't adapt any process which cause the source wavelet distortion and amplitude variation. Main data processing flow are

SEG-D Field Data Loading

Geometry Input

True Amplitude Recovery(TAR)

Static Correction

Predictive Deconvolution

Wave Equation Multiple Rejection(WEMR)

Radon Filtering

Dip Moveout Correction)

Interactive Velocity Analysis

We've strongly focused on getting rid of ambient noise and multiple and set the most appropriate parameters at each processing step by a lot of test and analysis. RMS velocity field was used as initial velocity model for AVO inversion.

And AVO analysis processes we've followed in this study are

Summary of AVO theory

Rearrange the common source gather to common angle gather

Amplitude analysis in common offset domain

Amplitude analysis in time slice window

AVO attributes stack and generate the stack section

AVO attributes cross plot

AVO anomaly analysis

Rock-physical attributes calculation and cross plot using AVO inversion

Generate AI map, High resolution interval velocity field using AVO inversion

The results of each seismic line

Line 92-6023

This seismic line crosses the exploration well Gorae V. According to the stack section, AVO anomaly was detected in the zone CDP 2400-2900, 3km depth and 1000-1900ms of the two-way travel time.

After interpreting the common offset section, Reflection horizon could not detect upto 500m offset but after that amplitude become bigger and bigger according to increasing the offset. Maximum amplitude could be seen at 1800m offset.

We made 9 time slices with every 20ms interval at target reflection horizon in order to analyze amplitude variation of the three zones, A, B, C. It was strongly confirmed that the amplitude variation of zone C is AVO anomaly by natural gas response.

Comparing with other reflection horizon, it couldn't be seen any outstanding AVO anomaly on the AVO attribute stack section. This phenomena could be estimated the characteristic of the natural gas bearing sediment zone in block 6-1. So more intensive analysis and considerable research is needed to obtain the reasonable conclusion.

After analysing AVO attribute response of the sand body bearing oil and natural gas, All 4 types of sand body was classified. Seismic line 92-6023 showed strong response for type 3 and type 4. Generally this sand body type could be thought as younger generation, saturated with brine, has high porosity.

Also acoustic impedance anomaly of this seismic line was quite differ with that of other seismic line. This acoustic impedance anomaly was caused by higher variation of S wave than that of P wave.

Using intercept and gradient, we calculated the high resolution interval velocity field. The resolution size of the interval velocity field is 6.25m laterally same of CDP interval and 1ms vertically. The interval velocity of target zone increased by 5000ms and then decreased by 4000ms nearby natural gas zone

We computed elastic module μρ,λρ using AVO attributes then crossploted these two value. After that we processed sesimic data in order to confirm attributes of target layers. Since S wave velocity picking is very hard not easy, we couldn't obtain high-quality results. So we concluded that a precise S velocity is absolutely critical in AVO inversion.

AVO Modeling Parameter of the Line 92-6023

The coherency between field seismic data and synthetic seismic data is 0.7125.

P wave velocity : 78.18 us/ft

S wave velocity : 144 ms/ft

Density: 2.42 g/cc

Possion’s ratio :    0.2945

Calculated input porosity : 11.04%

Calculated dry rock possion’s ratio : 0.2939

Water saturation: 50%

Land Seismic Data

2 land seismic data(B-04-07, B-04-47) from Bolz block in Kazakhstan were studied in the study to analyze whether seismic wave can travel under the salt body or not. Sub-salt ray trace modeling was chosen for the first method we undergone and then compared with the ray density both salt body and sediment layer.

According to the ray trace modeling results, a few of ray could travel under the salt body. Also, although experiment was done using synthetic model, in case that reservoir locates under the salt body, a weak reflection event could be recorded.

For salt body developed structure such as Koblandy in Bolz block, Dip moveout should be taken because of steep dip of layers. Considering spherical divergence of elastic wave, gain correction for lateral and vertical direction also should be taken at true amplitude recovery stage.

In this study, we did seismic data processing to analyze AVO response and interpret for AVO anomaly for land seismic data. Then we selected the appropriate parameter at each processing step.

We applied the data processing flow obtained in this study to Bolz block in Kazakhstan. Since seismic wave distortion around salt body and complicated physical attributes variation, tiny S wave variation, It was analyzed that AVO response hardly be visible even though area of gas field.