Importance of Good Time to Depth Conversion method to Reduce GRV Uncertainties of Producing Fields in the South Sumatra Block”.

Abstract

As time goes by many cases of volumetric model from various field in South Sumatera Block calculation is not match with the reserve calculation after production decline (e.g. P/Z evaluation). Mostly problem cause limited and low quality of seismic data that cause non optimum calculation Gross Rock Volume (GRV). Beside seismic interpretation for Gross rock volume calculation itself depending on time to depth conversion. When we already have an optimum seismic interpretation, the key factor that have a big influence the gross rock volume calculation is time to depth conversion method. Velocity model is the bridge between time domain to depth domain to get an accurate gross rock volume. The velocity model should be estimated properly and accepted by geological model of surrounding area. This paper shows the application of many cases of GRV calculation method using interval velocity that can generate proper velocity model to decrease an uncertainties GRV calculation. Usually most of field structures in South Sumatera Block area were generated using single equation for time to depth conversion. This method is fast, simple, and fit for some area. But it also sometime has disadvantage if we have limited and low quality seismic and well data. The disadvantage is, it is not account lateral velocity variation to honor geological model of deposition system, so it is not reflecting lateral velocity variation in every layer of formation. The velocity in every location has the same velocity value n trend. The equation generates depth structure map that has almost similar pattern with the time structure map. To honor lateral velocity variation, another time to depth conversion method that suitable is using interval velocity. By using this method, the velocity of every layer was considered. The variety of velocity in each layer will consider the reservoir and lithology variation that usually presence in checkshot and correlated with seismic horizon. In South Sumatera, the interval velocity is change in Lower Palembang Formation, Lower Telisa Formation, and Baturaja Formation. The distribution of first interval velocity is depend on the isochrone of Lower Palembang. The distribution of the next interval velocity is also depending on the next isochrone. The thickness of each layer is the result of its interval velocity and isochrones. Therefore, the top of reservoir is depending on the thickness of each layer above the reservoir. This method successfully applied in various fields and some prospect such as matching map in Meta-Lica field production, update Temelat field map, Gunung Kembang field update, and reduce uncertainty reserve of Flamboyan prospect. The application of interval velocity shows a better gross rock volume calculation. For Lica and Gunung Kembang field show significant of increase gross rock volume and reserve calculation that impact adjustment production method from those two fields. While Temelat field show close reserve value with P/Z evaluation and make an optimization of development location. These results show that time to depth conversion using interval velocity is success to predict volumetric more accurate. Basically every method of time to depth conversion are unique and can suitable in some region area. But if you have a concerns to add geological concept in limited seismic data in generation velocity model, interval velocity model can be powerful to cover up those concepts.

AAPG Datapages/Search and Discovery Article #90354 © 2019 AAPG Asia Pacific Technical Symposium, The Art of Hydrocarbon Prediction: Managing Uncertainties, Bogor, (Greater Jakarta), Indonesia, August 7-8, 2019