Salt invasion in a Triassic reservoir. A case study from the Southern North Sea

Transcription

1 Salt invasion in a Triassic reservoir A case study from the Southern North Sea

2 Agenda Regional setting Triassic gas play Statistical investigation Seismic attribute analysis Conclusions & further study Acknowledgements Data presented with permission from EBN B.V., Dana Petroleum Netherlands B.V. and Wintershall Noordzee B.V Salt Tectonics Seminar, Stavanger 2

3 Plate Tectonic Reconstruction Late Permian - Early Triassic SIBERIA PANTHALASSIC OCEAN URAL CALEDONIAN PERMIAN BASIN VARISCAN PALEO-TETHYS OCEAN PANGEA From: Scotese, C.R., 2002, (PALEOMAP website) Salt Tectonics Seminar, Stavanger 3

4 Regional Setting LATE PERMIAN PALEOGEOGRAPHY SCYTHIAN PALEOGEOGRAPHY From: Ziegler (2005) Salt Tectonics Seminar, Stavanger 4

5 Regional Setting STUDY AREA 50 km Salt Tectonics Seminar, Stavanger 5

6 PSTM (msec) Regional Setting DUTCH CENTRAL GRABEN HANTUM FAULT TERSCHELLING BASIN TERTIARY CRETACEOUS JURASSIC TRIASSIC SALT PERMO/CARBONIFEROUS Salt Tectonics Seminar, Stavanger 6

7 Regional Setting Salt Tectonics Seminar, Stavanger 7

8 Triassic Gas Play Reservoir & Top-Seal L8-11 WELL Röt Fm. Solling Fm. Hardegsen Fm. Detfurth Volpriehausen Lower Bunter Claystone Fm. Volpriehausen Sandstone is main reservoir in study area Thickness: m Net/Gross: % Average porosity: 9-15 % Average permeability: md Depositional environment is continental under semi-arid conditions (eolian and fluvial sandstones with playa lake claystones) Röt Formation (with evaporites) is ultimate top seal In some cases Hardegsen or Volpriehausen Claystone can also be sealing Salt Tectonics Seminar, Stavanger 8

9 Triassic Gas Play Trapping Mechanisms & Charge Triassic structures are generally 4-way dip closures and gentle anticlines with only minor faulting, conformable with Top Salt Relying on windows through Salt for charge from Carboniferous coals Carboniferous mature at present day Salt Tectonics Seminar, Stavanger 9

10 Triassic Gas Play Reservoir Effectiveness Halite cement hardly visible on macro scale Dissolves in water-based drilling mud Often destroys all remaining porosity except micro-porosity in clay minerals Salt Tectonics Seminar, Stavanger 10

11 Statistical Investigation Possible causes for salt plugging in Triassic Authigenic cement present in sediments Infiltration from Röt Formation above Infiltration from Zechstein below Lateral invasion from pierced salt domes Precipitation from brine when uplifted (thermogenic) Combinations of all of the above Salt Tectonics Seminar, Stavanger 11

12 Average Porosity Average Porosity Average Porosity Average Porosity Statistical Investigation 25% 25% 20% TREND OF DECREASING POROSITY WITH DEPTH 20% LEACHING? 15% 15% 10% 10% 05% 05% 00% Depth Top Volpriehausen Sst. [m] 00% Vertical Distance below BCU [m] 25% 25% 20% 20% 15% 15% 10% 10% 05% 05% 00% Vertical Distance below Röt Evaporite [m] 00% Horizontal Distance to Salt Structure [km] Salt Tectonics Seminar, Stavanger 12

13 Seismic Attribute Analysis Lowest amplitude in lower time window minus the highest amplitude in upper time window: Normal porosity = = - 20 Salt plugged, good pick = = ~ 0 Salt plugged, wrong pick = = - 10 If porous and water/gas bearing, Top Volpriehausen is a peak, followed by a clear trough Presence of gas causes a brightening If salt-plugged, Top Volpriehausen is a trough, followed by a clear peak If a salt-plugged Volpriehausen is incorrectly picked, Top Volpriehausen is a clear peak, followed by low-amplitude events Supported by forward modelling Amplitude difference scaled to factor between 0 and Salt Tectonics Seminar, Stavanger 13

14 Seismic Attribute Analysis Scaled Amplitude Difference MB4 White circles indicate potential good reservoir Dashed circles indicate possible mediocre quality reservoir or areas with a wrong pick MB5 MB1 MB2 MB6 MB3 5 km MB7 SALT PLUGGED WELL POROUS WELL Salt Tectonics Seminar, Stavanger 14

15 Seismic Attribute Analysis Calibration to Wells SCALED PEAK-TROUGH AMPLITUDE AVERAGE POROSITY 0% 5% 10% 15% 20% 25% 1,00 0,90 0,80 0,70 0,60 0,50 0,40 0,30 0,20 SALT PLUGGED WELLS POROUS & DRY WELLS GAS FILLED WELLS Salt plugged wells (porosity < 10%) have high scaled amplitude values Porous wells (porosity > 10%) have medium to low scaled amplitude values Gas filled wells have low scaled amplitude values 0,10 0, Salt Tectonics Seminar, Stavanger 15

16 Seismic Attribute Analysis Scaled Amplitude Difference L2-FA L5-FA L9-FC???? M7-A Salt Tectonics Seminar, Stavanger 16

17 Possible Mechanism SALT DOME Cooling effect of salt dome can lead to precipitation of salt from brine ~ 5 km High thermal conductivity of salt results in cooling effect near salt dome Lateral temperature differences of up to 6 ºC measured Cooling of saturated brine results in salt precipitation Modelling study in preparation Salt Tectonics Seminar, Stavanger 17

18 Conclusions Salt plugging in Triassic reservoirs occurs pervasively over large areas or in rims (0.5 2 km) around salt domes or former halokinetic structures (collapse grabens) Seismic attributes can help to identify areas of salt plugging and gas fill Salt cement was leached during the Early Cretaceous when the reservoir was close to the surface, probably by invasion of meteoric waters Salt Tectonics Seminar, Stavanger 18

19 Further Study Can salt cementation result in a lateral side-seal, creating stratigraphic traps? Apply this tool to identify Triassic gas fields in other areas Are there any low-porosity Triassic gas fields? Test the method on other plays Salt Tectonics Seminar, Stavanger 19

20 Salt Tectonics Seminar, Stavanger 20