Integrating Rock Physics into the Seismic Image

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Cornelius Perry
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1 Government-Funded Exploration License Competition The project: Mid North Sea High Integrating Rock Physics into the Seismic Image Kinga Wroblewska, Geophysicist Geop4ysics Ltd Petex, 17 November 2016

2 Integrating Rock Physics into the Seismic Image for the MNSH project: Introduction: The presentation is going to focus on the Lower Triassic Bunter Sand Formation reservoir which can be considered as a secondary target in the MNSH area. The following aspects will be considered: structural settings reservoir property variability Seismic broadband inversion application to describe reservoir and seal extend

In some areas it is absent either because of erosion, or non-deposition.

3 Structural settings: The Bunter sandstone is present in greater thickness in the southern area of Quadrants 41, 42, 43 and 44, having a thickness up to 400 ft but significantly reduced in the eastern part of the MNSH. In some areas it is absent either because of erosion, or non-deposition. Erosion at the BCU resulted in removal of Bunter sand from crests of structural uplifts and swells caused by salt movement. W E Top Triassic Top Bunter Top Zechstein Top L. Carboniferous Base Zechstein 3 The seismic 2D composite line

4 Carboniferous is regarded as a gas-prone source rock for Triassic reservoirs. Two areas with significant Carboniferous thickness containing Namurian and Westphalian sediments are visible on the map in the SW part of the MNSH area (Quadrants 41,42,43 and 44). The centre of subsidence and the greatest thickness of Carboniferous strata occurs in zone A where Carboniferous and Bunter Triassic reservoirs were proven. The Esmond, Forbes and Gordon gas fields were documented as anticlines in Bunter Sand Formation and charged with gas most likely from the Carboniferous Westphalian source rocks. The map shows the main faults in the Carboniferous (zones A, B, C). B C A The Middle and Upper Carboniferous TWT thickness map interpreted in the Quadrants 41,42,43 and 44

5 This map presents the main fault zones interpreted at the BSF horizon. The main faults zones present in Mesozoic sediments are correlateable with the main Carboniferous fault lineaments (zones B and C), however the Zechstein evaporites can act as a detachment between younger Mesozoic and pre-zechstein deposits. The fault zones can give some guidance about the possible zones of a gas leakage from Westphalian source rocks. Folding during the Cretaceous compression, the salt movement and withdrawal during Mesozoic and later times induced the development of large synclines and anticlines in the Bunter formation. However, both the 2D legacy and new seismic data are too widely spaced to map either fault zones or structural closures in the Bunter reservoir with confidence. B C A The TWT top Bunter map calculated from sparse 2D profiles

Reservoir property variability: The correlation panel with the main wells shows sonic slowness and gamma logs with well tops flattened at Top Bacton.

6 Reservoir property variability: The correlation panel with the main wells shows sonic slowness and gamma logs with well tops flattened at Top Bacton. The quality and thickness of Bunter sand varies over this huge area, with NW wells (e.g. 41/05-1) having thicker (130 m) sand compared with poorer quality and thinner sand in the 42/10a-1well (18 m). The quality of the Bunter sand reservoir can be related to AI values. The better reservoir properties of the sand in 44/12-1 are related to lower sonic velocity. The 42/10a-1 and 43/06-1 wells where halite and anhydritic cemented sand was encountered have high AI values in the BSF. The Haisborough Group with Rot Halite is seen to form the primary seal to the Bunter sand along with the Muschelkalk and Keuper in the higher part of the Haisborough section. SE NW

7 These two seismograms show the seismic response for the BSF reservoir in 42/10a-1 with a poor quality thin sand, and 44/12-1 with a thicker and better quality reservoir. The Top Bacton event was recognized as having a different polarity and it was interpreted as being a negative or positive reflector. 42/10a-1 44/12-1

Seismic inversion application and analysis: To understand the changes in seismic amplitude and relate them to the rock properties, a seismic inversion study was conducted.

8 Seismic inversion application and analysis: To understand the changes in seismic amplitude and relate them to the rock properties, a seismic inversion study was conducted. Two different algorithms were applied: - Coloured Inversion to obtain Relative Acoustic Impedance - Model Based Inversion in order to obtain Absolute Acoustic Impedance The advantage of broadband data over the legacy seismic should be important especially in areas with limited well control. The broadband seismic should represent a wide spectrum with high and low frequencies to be able to perform an accurate Absolute Impedance calculation. However, the low frequency model is still needed along with a robust broadband wavelet extracted from wells and seismic. The diagram shows the QC Inversion analysis in 38/24-1 well for a 350 ms broadband wavelet and a low frequency model (4/8 Hz) for the seismic 18A516 line Low frequency model AI log Inverted seismic

9 The observed faults at the seismic 43A574 line have caused gas leakage as a gas chimney is visible through the Triassic and the Chalk section. Top Triassic Top Bacton Top Zechstein Base Zechstein Top Namurian

10 Seismic line 43A574 - Absolute Impedance The Bunter Sand Formation shows low Acoustic Impedance values. High acoustic impedance was extracted in the overburden.

11 43A574 Line; A - Near and B - Far stack. Far stacks display high amplitude at Bunter Sand Formation A B

Low AI was derived in the inversion process for BSF.

12 47A524 seismic line (Absolute Impedance). A Thick and good quality reservoir was encountered in 44/12-1. Low AI was derived in the inversion process for BSF. Wireline logs show a good correlation in the Bacton sand and in the overburden. The Bacton seal the Haisborough group contains mudstones, Rot Halite and dolomitic beds with higher AI values. Sonic Velocity

13 The inversion QC was conducted by a blind test at 42/10a-1 for line 47A524. The validation error analysis shows the good correlation at the well location between the Acoustic Impedance profile and the AI log. The 18 m thick Bunter sand that was encounter in the well contains halite cement and has very poor porosity.

14 Summary: The application of broadband seismic inversion can be a valuable tool in less explored areas where there is little well control. The AI model was created from a sparse well data and the low frequency filter was applied. Broadband data can deliver a wider frequency spectrum compared with a conventional seismic and give good correlation in the QC process for seismic inversion to delineate BSF reservoir.

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