Petroleum System Trip in Cepu Field by UGM Student Chapter

American Association of Petroleum Geologists (AAPG)

The activity was held in two days field trip with direct method come to field. The Fieldtrip was held on Sunday and Monday at December, 3rd – 4th 2006 in Cepu,Central Java. The title is “Petroleum System Trip in Cepu Field”. This is the activities each stop sites :

Stop site 1 : Mrapen, Purwodadi

This stop site is located in road side purwodadi – semarang, aparting 26 km on the west from town Purwodadi. Precisely in countryside of Manggarmas, subdistrict Godong, Grobogan regency or 60 kilometers east of Semarang.

At this stop site there is flame comes out from the ground under piles of enflamed rocks like a fireplace. The flame never dies even in heavy rain. In fact, this eternal flame is a gas that came out through the gap that haved some path and it’s formed by geologycal structure such as fault, and this gas become a fire because somebody have turn the fire on. The producer of this gas is gs seepage in the subsurface.

From the research of Lemigas Departement, this gas is predicted as a biogenic gas, that is produced by biopolymerytation process. And this gas dominated by mthane ( CH4 ), but this gas is harmless because the precentage of the gas which came out to the surface is so small.

Not far from eternal flame location, there is water pool which call Sendang Dudo. The water in this pool is like boilling water with bubble came out to the surface of the water, it’s because of the gas presure from underground. The gas that came out from the water has same process as the gas in internal flame, it’s mean that both of this gas have same source or same producer.

There is many event that used eternal flame from Mrapen. For example, Mrapen is used for preparations of the Waisak ceremony commence. The procession to Borobudur to celebrate Siddharta Gautama’s birthday begins by lighting the flame at Mrapen. The flame is used to give light in the evening during the ceremony. It is also used to light the torch for PON (National Sports Competition).

And for the common knowledge we will give the legend story of Mrapen. According to legend, the flame was used by a powerful empu Supo (maker of keris, the wavy double-bladed dagger) to melt the metal to make keris by using his supernatural power, Sendang Dudo is for Gilding, and Watu Bobot is for the place to forge the patrimony keris. Empu Supo was known as Sunan Kalijaga’s relative and was an important figure in spreading Islam in Central Java in the 15th century.

Stop site 2 : Bledug Kuwu, Purwodadi

The journey Purwodadi to Ngampel acros Randublatung depretion zone in the form of plain till undulating morphology which medium in fact represent the fold notching which not be strong so. Others, will be met some of Rembang zona in the form of anticlinorium.

At this stop site, we will one of experienced phenomenon in the form of pregnant eruption sulphur gas and clay materials. This phenomenon known as mud volkano. Gas come from organic material or global genic gas. The water that came out is called conate water. This water trapped at the same time with the rock depotititon and olso can be crystalization become salt. Later, this process become one of living for population around. While value salinity used for the determination of depotition environmental.

Caused by subsurface preasure and gas drive accumulate from earth making mud impelled a exit. Influence of Meteoric water just to making mud become mellowier and assist so conate water can carried it out within eruption.

Others, the forming of this mud volcano related / relevant also with the process of seal diaphiric. Clay materials represent the existing material from formation infiltrated by conate water. Acording to existing characteristic it may be come from Lidah formation. There is no definitive research so it might possibly from Mundu formation, Wonocolo formation, or Ngrayong formation.

Clay Volume yielded possible experience of the accretion but not significan or its volume represent the constant volume.

Differ from the mud volkano exist in Sangiran area, in this area the mud volcano have passive and its own clay exploited for the making of gerabah.

Local people believe that the source of the salt water is the Indian Ocean that flows underground to the area of Bleduk Kuwu. Why is this? It has a connection with a legend called “ Aji Soko “ recorded in a book called “ Babad Tanah Jawi “. This book is kept in the Radya Pustaka Museum in Surakarta.

The legend is about a gigantic snake called Joko Linglung. He came to see Aji Soko, the king of Medang Kamulan. This snake confessed that he was Aji Soko’s son. Aji Soko was thought to come from India and was King Suriti’s son. He then became the king of Medang Kamulan after killing Prabu Dewata Cengkar, the former king of Medang Kamulan. Aji Soko would admit Joko Linglung as his son if he could kill the white crocodile that lived in the Indian Ocean. The white crocodile was believed to be the reincarnation of Prabu Dewata Cengkar.

At the end of the story, Joko Linglung was successful in killing the white crocodile and then went to see Aji Soko. On his way back home he went underground through villages like Yono, Banjarsari, Crewek and finally appeared in Kuwu because he got lost. That’s why the place is known as Desa Kuwu where the little mud explosions of salt water are found.

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