Client: PSG – Indonesia

Scope of the survey was to:

  • Obtain an accurate velocity model
  • Obtain a 3D geological model
  • Locate possible targets of hydrocarbon potential

The block is located the Central Range at SE Papua (aka Highlands fold-and-thrust belt) which was formed by two tectonic events: the first involved obduction of the Papuan ophiolites during Oligocene and caused the metamorphism of the continental margin sediments, and the second is coeval to the Pliocene to Present arc-continent collision, demonstrating ramp anticline features and flat-ramp thrusting due to the SW-NE-trending vengeance, in addition to shear.

The installed PST network. Due to heavy jungle in some areas it was not possible t install stations but an alternative seismic array data processing methodology was used to cover these “holes”.

 

Due to the muddy environment and the high tide in the study area, an extra care was taken for the installation of the seismic stations constructing special platforms. In addition, to maximize the signal to noise ratio (SNR) by reducing the effect of the movement of the trees, the weather conditions and the sea waves, all seismometers were installed in boreholes that were reaching down to a depth of 18 m.

Basic principles of construction of a typical PST station in this project

The located seismicity is depicted below. From the location of the microseismic events, it is obvious that the study area is characterized by high seismicity rate. The depths of the seismicity are generally shallow and this is also compatible with the young morphology since the Tarera-Aiduna transcurrent fault zone (TAFZ) is part of the new active Pacific-Australia plate boundary. Most of the events occurred at the seismogenic area along TAFZ.

The microseismic events recorded between June, 23 and October 22, 2016, as located by the use of a specially designed velocity model.

The station corrections for the 1-D minimum P-wave velocity model

The station corrections for the 1-D minimum S-wave velocity model

The construction of the 3D velocity structure using LandTech’s latest Tomographic inversion software

Prior to the interpretation many quality control tests have been performed to insure the accuracy of the results. Before the detailed presentation of the results we should clarify that the confidence of the model differs from place to place according mainly to the ray-coverage distribution. This ray coverage is given by the hit counts and their distribution. The distribution of hit counts shows the number of the rays passing through each grid node
(hit count is the sum of the number of rays that contribute to the solution at one node). The figure below shows that the hit counts limit is good even down to 20 km depth.

Hit counts distribution is a direct image of the ray coverage

On the other hand, the derivative weighted sum (DWS) shows (see figure below) the ray density in the neighbour area of a model node. Ray density is a measure of the normalized total length of rays in a unit volume. In other words, DWS is a measure of the density of the seismic rays at a velocity node neighbourhood area.

The DWS iso-surface of value 260 covers the most of the study area down to 20 km depth.

After examining the horizontal tomographic slices we can notice the existence of predominant linear features, apparently associated with major tectonic structures. In we can notice the anomaly indicated by the gray-coloured arrow. We further investigated this anomaly by examining also the 3D velocity structure using the PAT tomographic software.


Linear anomalies can be associated with the major tectonic structures of the study area. The horizontal slice is taken at a depth of 7 km.

We found that this anomaly can be related with an underthrust structure that raptures and deforms the upper layers.

The 3D view of the anomaly revealed that this can be associated with n underthrust structure

One of the most important tectonic structures in the area has been identified by the PST survey. The western tip of the Aiduna Fault is clearly depicted in the horizontal slices of the geophysical investigation due to the horizontal displacement juxtaposing formations of different properties and seismic velocity. Moreover, the normal sense of motion is also evident from the low-velocity sediments (noted by A and B in figure below) in south of the Buru Mountains, which are accommodated in the tectonically controlled basin (hangingwall).

The major tectonic faults number from 1-7 indentified by the PST survey at the horizontal slice of 7 km depth

 

 

 

Below we present Vp/Vs Horizontal slices as calculated by PST

 

The limestone layer can be shown with green colour

At an average depth of about 8 km and deeper, below the siliclastics the velocity increases over the limit of 6.0 km/s and the Vp/Vs increases. This is an indication of the presence of the dolomitic Modio formation at those depths (and the velocity progressively reaches values up to 6.7 km/s.

 


The border of the Dolomite formation

Scope of the survey was to:

  • Obtain an accurate velocity model
  • Obtain a 3D geological model
  • Locate possible targets of hydrocarbon potential