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100% SUCCESS IN PREDICTION SUBSALT GAS RESERVOIRS CONFIRMED BY DECADES OF EXPLORATION DRILLING


Solotvyno and Dibrova gas fields,
Transcarpathian Trough,
Ukraine, 2005




Table 1. Drilling history and well testing results in Dibrova and Solotvyno gas fields

GEOLOGICAL PROBLEM

The study area belongs to Transcarpathian Trough which is Miocene molasse basin,  underlayed by Paleogene-Mezozoic basement. In the south-eastern part of the study area salt pierces Neogene clastic sequence and forms Solotvyno salt diapir. The top and the bottom of salt diapir can not be mapped by seismic methods due to alteration of salty and clastic beds.
 
Solotvyno gas field is situated in the eastern part of the study area. Gas pools were discovered by wells #1 and #2 and are confined to Lower Badenian tuffs of Novoselytsya formation N1nv. Dibrova gas field is situated in the central part of the study area. Gas pools were discovered by wells #4 and #22 and are confined to Paleogene sediments P3gr.

Both fields are located near Solotvyno salt diapir and are overlaid by salty beds of Tereblya formation. 

By 2D seismic data, Solotvyno and Dibrova gas pools occure in Neogene and Paleogene anticlinal structures respectively. Wells #4 and #22 of Dibrova field were placed in northern pericline of Dibrova anticline; wells #1 and #2 - in western pericline of Solotvyno anticline.

 
The main objective of the study was to refine the shape of Solotvyno salt diapir and to delineate distribition of gas-saturated reservoirs, discovered by producing wells of Solotvyno and Dibrova fields.

 Figure 1. The contour of gas-saturated reservoir in Paleogene of Dibrova field outlined by density value of 2.50 g/ccm. Dry wells №23 and №28 discovered dense impermeable rocks

Figure 2.The contour of gas-saturated reservoir in Neogene of Solotvyno field outlined by density value of 2.38 g/ccm. Producing well №15 discovered low-density gas-saturated rocks

GEOLOGICAL RESULTS

The shape of Solotvyno salt diapir was refined by results of 3D gravity inversion. It turned out that the bottom of salt body deeps over  2.5 km down while in the initial 3D model it reached only 1.3 km depth (Figure 3).

Paleogene gas pool of Dibrova field corresponds to low-density area outlined by maximum density value of 2.50 g/ccm. Producing wells #4 and #22 are located within the low-density anomaly (Figure 1). The most qualitative gas-saturated reservoirs are developed to the south from wells #4 and #22 under Solotvyno salt diapir.

The rest of the anticlinal structure is characterized by dense sediments which indicates an absence of good reservoirs and hydrocarbon saturation. It was confirmed by dry wells #23 and #28 (Table 1).

Neogene gas pool of Solotvyno field corresponds to low-density area outlined by maximum density value of 2.38 g/ccm. Producing wells #1 and #2 are located within the low-density anomaly (Figure 2). Neogene gas pool is located in the apical and south-eastern part of Solotvyno anticline. Gas inflow from well #15 within the apical part of the structure (Table 1) confirms validity of the outlined contour of Neogene gas pool in Solotvyno field.

Figure 3. Initial (а) and resulting (b) 3D density models

3D GRAVITY INVERSION WORKFLOW
 
3D structural framework was built using 20 2D seismic lines, as well as stratigraphic well tops from different wells including #4 and #22 of Dibrova field and #1 and #2 of Solotvyno field. Structural model consisted of 7 surfaces, featuring the structure of Neogene and Paleogene. The initial 3D density model (Figure 3,a) was built using generalized petrophysical relations. Dimensions of the 3D density model were 14.4x10x7 km. Voxel property model discretization (cell size) - 100x100x50 m. Total number of cells – 2 million.
Sandard deviation (SD) between observed gravity and gravity calculated from the initial 3D density model was 3.792 mGal (Figure 4,d). The first stage of applied algorithm - 3D non-linear gravity inversion led to refining the shape of salt diapir and reducing deviation between observed and calculated gravity to 1.5 mGal (initial 3D density model was improved by 2.5 times). The next stage - 3D linear gravity inversion led to refining 3D density model and reducing deviation between observed and calculated gravity to 0.316 mGal (relative to the gravity field, the initial 3D density model was improved by 12 times) (Figure 4,e).

Figure 1. Observed (а) and calculated gravity fields in case of initial (b) and resulting (с) 3D density models and gravity misfit in case of initial (d) and resulting (e) 3D density models

PUBLICATIONS

1. Петровський О.П., Федченко Т.О. Нові напрями застосування даних акустичних досліджень в свердловинах для відновлення неоднорідної сейсмошвидкісної моделі середовища та сейсмо-стратиграфічного розчленування розрізу Солотвинської площі. Геоінформатика. 2006. - №3. - С.29-33 

2. Федченко Т.О.,  Петровский О.П. Пространственная информативность априорных геолого-геофизических данных и эффективность интегрального моделирования нефтегазо-перспективных объектов. Геофизический журнал. 2010. - №3. Т.32 - С.125-135

 

3. Fedchenko T. , Petrovskyy O., Trachuk A., Onyschuk O. Subsalt 3-D Modelling and HC Reservoir Prediction With Scarce 2-D Seismic Datasets: Can We Obtain Reliable Results? AAPG Annual Conference & Exhebition, Houston, on 02-05 April 2017, Poster presentation.

 

 

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